diff --git a/lib/mv/membership/import/header_mapper.ex b/lib/mv/membership/import/header_mapper.ex
index caf7245..4e4a77d 100644
--- a/lib/mv/membership/import/header_mapper.ex
+++ b/lib/mv/membership/import/header_mapper.ex
@@ -97,17 +97,33 @@ defmodule Mv.Membership.Import.HeaderMapper do
   }
 
   # Build reverse map: normalized_variant -> canonical_field
-  # This is computed at runtime on first access and cached
+  # Cached on first access for performance
   defp normalized_to_canonical do
+    cached = Process.get({__MODULE__, :normalized_to_canonical})
+
+    if cached do
+      cached
+    else
+      map = build_normalized_to_canonical_map()
+      Process.put({__MODULE__, :normalized_to_canonical}, map)
+      map
+    end
+  end
+
+  # Builds the normalized variant -> canonical field map
+  defp build_normalized_to_canonical_map do
     @member_field_variants_raw
-    |> Enum.flat_map(fn {canonical, variants} ->
-      Enum.map(variants, fn variant ->
-        {normalize_header(variant), canonical}
-      end)
-    end)
+    |> Enum.flat_map(&map_variants_to_normalized/1)
     |> Map.new()
   end
 
+  # Maps a canonical field and its variants to normalized tuples
+  defp map_variants_to_normalized({canonical, variants}) do
+    Enum.map(variants, fn variant ->
+      {normalize_header(variant), canonical}
+    end)
+  end
+
   @doc """
   Normalizes a CSV header string for comparison.
 
@@ -208,9 +224,10 @@ defmodule Mv.Membership.Import.HeaderMapper do
     |> String.replace(<<0x2212::utf8>>, "-")
   end
 
-  # Normalizes punctuation: parentheses, slashes become spaces
+  # Normalizes punctuation: parentheses, slashes, underscores become spaces
   defp normalize_punctuation(str) do
     str
+    |> String.replace("_", " ")
     |> String.replace(~r/[()\[\]{}]/, " ")
     |> String.replace(~r/[\/\\]/, " ")
   end
@@ -228,18 +245,18 @@ defmodule Mv.Membership.Import.HeaderMapper do
     result =
       headers
       |> Enum.with_index()
-      |> Enum.reduce_while({%{}, [], %{}}, fn {header, index}, {acc_map, acc_unknown, acc_seen} ->
+      |> Enum.reduce_while({%{}, []}, fn {header, index}, {acc_map, acc_unknown} ->
         normalized = normalize_header(header)
 
-        case process_member_header(header, index, normalized, acc_map, acc_seen) do
+        case process_member_header(header, index, normalized, acc_map, %{}) do
           {:error, reason} ->
             {:halt, {:error, reason}}
 
-          {:ok, new_map, new_seen} ->
-            {:cont, {new_map, acc_unknown, new_seen}}
+          {:ok, new_map, _} ->
+            {:cont, {new_map, acc_unknown}}
 
           {:unknown} ->
-            {:cont, {acc_map, [index | acc_unknown], acc_seen}}
+            {:cont, {acc_map, [index | acc_unknown]}}
         end
       end)
 
@@ -247,7 +264,7 @@ defmodule Mv.Membership.Import.HeaderMapper do
       {:error, reason} ->
         {:error, reason}
 
-      {member_map, unknown_indices, _normalized_seen} ->
+      {member_map, unknown_indices} ->
         validate_required_fields(member_map, unknown_indices)
     end
   end
@@ -258,17 +275,17 @@ defmodule Mv.Membership.Import.HeaderMapper do
     {:ok, acc_map, acc_seen}
   end
 
-  defp process_member_header(_header, index, normalized, acc_map, acc_seen) do
-    if Map.has_key?(normalized_to_canonical(), normalized) do
-      canonical = normalized_to_canonical()[normalized]
+  defp process_member_header(_header, index, normalized, acc_map, _acc_seen) do
+    case Map.get(normalized_to_canonical(), normalized) do
+      nil ->
+        {:unknown}
 
-      if Map.has_key?(acc_map, canonical) do
-        {:error, "duplicate header for #{canonical} (normalized: #{normalized})"}
-      else
-        {:ok, Map.put(acc_map, canonical, index), Map.put(acc_seen, normalized, canonical)}
-      end
-    else
-      {:unknown}
+      canonical ->
+        if Map.has_key?(acc_map, canonical) do
+          {:error, "duplicate header for #{canonical} (normalized: #{normalized})"}
+        else
+          {:ok, Map.put(acc_map, canonical, index), %{}}
+        end
     end
   end
 
@@ -295,7 +312,7 @@ defmodule Mv.Membership.Import.HeaderMapper do
 
     result =
       unknown_indices
-      |> Enum.reduce_while({%{}, [], %{}}, fn index, {acc_map, acc_unknown, acc_seen} ->
+      |> Enum.reduce_while({%{}, []}, fn index, {acc_map, acc_unknown} ->
         header = Enum.at(headers, index)
         normalized = normalize_header(header)
 
@@ -305,16 +322,16 @@ defmodule Mv.Membership.Import.HeaderMapper do
                normalized,
                custom_field_lookup,
                acc_map,
-               acc_seen
+               %{}
              ) do
           {:error, reason} ->
             {:halt, {:error, reason}}
 
-          {:ok, new_map, new_seen} ->
-            {:cont, {new_map, acc_unknown, new_seen}}
+          {:ok, new_map, _} ->
+            {:cont, {new_map, acc_unknown}}
 
           {:unknown} ->
-            {:cont, {acc_map, [index | acc_unknown], acc_seen}}
+            {:cont, {acc_map, [index | acc_unknown]}}
         end
       end)
 
@@ -322,7 +339,7 @@ defmodule Mv.Membership.Import.HeaderMapper do
       {:error, reason} ->
         {:error, reason}
 
-      {custom_map, remaining_unknown, _normalized_seen} ->
+      {custom_map, remaining_unknown} ->
         {:ok, custom_map, Enum.reverse(remaining_unknown)}
     end
   end
@@ -350,10 +367,10 @@ defmodule Mv.Membership.Import.HeaderMapper do
          normalized,
          _custom_field_lookup,
          acc_map,
-         acc_seen
+         _acc_seen
        )
        when normalized == "" do
-    {:ok, acc_map, acc_seen}
+    {:ok, acc_map, %{}}
   end
 
   defp process_custom_field_header(
@@ -362,7 +379,7 @@ defmodule Mv.Membership.Import.HeaderMapper do
          normalized,
          custom_field_lookup,
          acc_map,
-         acc_seen
+         _acc_seen
        ) do
     if Map.has_key?(custom_field_lookup, normalized) do
       custom_field_id = custom_field_lookup[normalized]
@@ -370,8 +387,7 @@ defmodule Mv.Membership.Import.HeaderMapper do
       if Map.has_key?(acc_map, custom_field_id) do
         {:error, "duplicate custom field header (normalized: #{normalized})"}
       else
-        {:ok, Map.put(acc_map, custom_field_id, index),
-         Map.put(acc_seen, normalized, custom_field_id)}
+        {:ok, Map.put(acc_map, custom_field_id, index), %{}}
       end
     else
       {:unknown}
diff --git a/lib/mv/membership/import/member_csv.ex b/lib/mv/membership/import/member_csv.ex
index 7790bff..26756b4 100644
--- a/lib/mv/membership/import/member_csv.ex
+++ b/lib/mv/membership/import/member_csv.ex
@@ -63,6 +63,7 @@ defmodule Mv.Membership.Import.MemberCSV do
           chunks: list(list({pos_integer(), map()})),
           column_map: %{atom() => non_neg_integer()},
           custom_field_map: %{String.t() => non_neg_integer()},
+          custom_field_lookup: %{String.t() => %{id: String.t(), value_type: atom()}},
           warnings: list(String.t())
         }
 
@@ -118,11 +119,15 @@ defmodule Mv.Membership.Import.MemberCSV do
          :ok <- validate_row_count(rows, max_rows) do
       chunks = chunk_rows(rows, maps, chunk_size)
 
+      # Build custom field lookup for efficient value processing
+      custom_field_lookup = build_custom_field_lookup(custom_fields)
+
       {:ok,
        %{
          chunks: chunks,
          column_map: maps.member,
          custom_field_map: maps.custom,
+         custom_field_lookup: custom_field_lookup,
          warnings: warnings
        }}
     end
@@ -140,6 +145,15 @@ defmodule Mv.Membership.Import.MemberCSV do
       {:error, "Failed to load custom fields: #{Exception.message(e)}"}
   end
 
+  # Builds custom field lookup map for efficient value processing
+  defp build_custom_field_lookup(custom_fields) do
+    custom_fields
+    |> Enum.reduce(%{}, fn cf, acc ->
+      id_str = to_string(cf.id)
+      Map.put(acc, id_str, %{id: cf.id, value_type: cf.value_type})
+    end)
+  end
+
   # Builds header maps using HeaderMapper and collects warnings for unknown custom fields
   defp build_header_maps(headers, custom_fields) do
     # Convert custom fields to maps with id and name
@@ -263,11 +277,13 @@ defmodule Mv.Membership.Import.MemberCSV do
           %{String.t() => non_neg_integer()},
           keyword()
         ) :: {:ok, chunk_result()} | {:error, String.t()}
-  def process_chunk(chunk_rows_with_lines, _column_map, _custom_field_map, _opts \\ []) do
+  def process_chunk(chunk_rows_with_lines, _column_map, _custom_field_map, opts \\ []) do
+    custom_field_lookup = Keyword.get(opts, :custom_field_lookup, %{})
+
     {inserted, failed, errors} =
       Enum.reduce(chunk_rows_with_lines, {0, 0, []}, fn {line_number, row_map},
                                                         {acc_inserted, acc_failed, acc_errors} ->
-        case process_row(row_map, line_number) do
+        case process_row(row_map, line_number, custom_field_lookup) do
           {:ok, _member} ->
             {acc_inserted + 1, acc_failed, acc_errors}
 
@@ -280,9 +296,13 @@ defmodule Mv.Membership.Import.MemberCSV do
   end
 
   # Processes a single row and creates member with custom field values
-  defp process_row(%{member: member_attrs, custom: custom_attrs}, line_number) do
+  defp process_row(
+         %{member: member_attrs, custom: custom_attrs},
+         line_number,
+         custom_field_lookup
+       ) do
     # Prepare custom field values for Ash
-    custom_field_values = prepare_custom_field_values(custom_attrs)
+    custom_field_values = prepare_custom_field_values(custom_attrs, custom_field_lookup)
 
     # Create member with custom field values
     member_attrs_with_cf =
@@ -314,53 +334,26 @@ defmodule Mv.Membership.Import.MemberCSV do
   end
 
   # Prepares custom field values from row map for Ash
-  defp prepare_custom_field_values(custom_attrs) when is_map(custom_attrs) do
+  defp prepare_custom_field_values(custom_attrs, custom_field_lookup) when is_map(custom_attrs) do
     custom_attrs
     |> Enum.filter(fn {_id, value} -> value != nil && value != "" end)
     |> Enum.map(fn {custom_field_id_str, value} ->
-      # Load custom field to get value_type and ensure ID is correct
-      case load_custom_field_by_id(custom_field_id_str) do
-        {:ok, custom_field} ->
-          # Use the actual custom_field.id (UUID) from the database
-          %{
-            "custom_field_id" => to_string(custom_field.id),
-            "value" => format_custom_field_value(value, custom_field.value_type)
-          }
-
-        {:error, _} ->
-          # Skip if custom field not found
+      case Map.get(custom_field_lookup, custom_field_id_str) do
+        nil ->
+          # Custom field not found, skip
           nil
+
+        %{id: custom_field_id, value_type: value_type} ->
+          %{
+            "custom_field_id" => to_string(custom_field_id),
+            "value" => format_custom_field_value(value, value_type)
+          }
       end
     end)
     |> Enum.filter(&(&1 != nil))
   end
 
-  defp prepare_custom_field_values(_), do: []
-
-  # Loads a custom field by ID (string or UUID)
-  defp load_custom_field_by_id(id) when is_binary(id) do
-    require Ash.Query
-
-    try do
-      # Try to parse as UUID first
-      uuid_id =
-        case Ecto.UUID.cast(id) do
-          {:ok, uuid} -> uuid
-          :error -> id
-        end
-
-      custom_field =
-        Mv.Membership.CustomField
-        |> Ash.Query.filter(id == ^uuid_id)
-        |> Ash.read_one!()
-
-      {:ok, custom_field}
-    rescue
-      _ -> {:error, :not_found}
-    end
-  end
-
-  defp load_custom_field_by_id(_), do: {:error, :invalid_id}
+  defp prepare_custom_field_values(_, _), do: []
 
   # Formats a custom field value according to its type
   # Uses _union_type and _union_value format as expected by Ash
diff --git a/test/mv/membership/import/member_csv_test.exs b/test/mv/membership/import/member_csv_test.exs
index a38315f..356cc19 100644
--- a/test/mv/membership/import/member_csv_test.exs
+++ b/test/mv/membership/import/member_csv_test.exs
@@ -72,7 +72,7 @@ defmodule Mv.Membership.Import.MemberCSVTest do
     end
   end
 
-  describe "process_chunk/3" do
+  describe "process_chunk/4" do
     test "function exists and accepts chunk_rows_with_lines, column_map, custom_field_map, and opts" do
       chunk_rows_with_lines = [{2, %{member: %{email: "john@example.com"}, custom: %{}}}]
       column_map = %{email: 0}
@@ -173,7 +173,12 @@ defmodule Mv.Membership.Import.MemberCSVTest do
 
       column_map = %{email: 0}
       custom_field_map = %{to_string(custom_field.id) => 1}
-      opts = []
+
+      custom_field_lookup = %{
+        to_string(custom_field.id) => %{id: custom_field.id, value_type: custom_field.value_type}
+      }
+
+      opts = [custom_field_lookup: custom_field_lookup]
 
       assert {:ok, chunk_result} =
                MemberCSV.process_chunk(chunk_rows_with_lines, column_map, custom_field_map, opts)