diff --git a/lib/microwaveprop/weather.ex b/lib/microwaveprop/weather.ex
index ac099a44..3b62dda2 100644
--- a/lib/microwaveprop/weather.ex
+++ b/lib/microwaveprop/weather.ex
@@ -466,9 +466,11 @@ defmodule Microwaveprop.Weather do
       :miss ->
         case ProfilesFile.read(valid_time) do
           {:ok, grid_data} ->
-            grid_data
-            |> build_grid_cache_rows(valid_time)
-            |> filter_weather_bounds(bounds)
+            # Filter-before-derive: only derive the points inside the
+            # viewport instead of all 92k CONUS grid points. On a DFW
+            # viewport that's ~20× less `SoundingParams.derive` work
+            # per timeline scrub.
+            build_grid_cache_rows(grid_data, valid_time, bounds)
 
           {:error, _} ->
             []
@@ -599,13 +601,22 @@ defmodule Microwaveprop.Weather do
   output matches the shape of `load_weather_grid_from_db/1` after
   `derive_and_clean/1`.
   """
-  @spec build_grid_cache_rows(%{{float(), float()} => map()}, DateTime.t()) :: [map()]
-  def build_grid_cache_rows(grid_data, valid_time) do
-    Enum.flat_map(grid_data, fn {{lat, lon}, profile} ->
+  @spec build_grid_cache_rows(%{{float(), float()} => map()}, DateTime.t(), map() | nil) ::
+          [map()]
+  def build_grid_cache_rows(grid_data, valid_time, bounds \\ nil) do
+    grid_data
+    |> filter_grid_data_bounds(bounds)
+    |> Enum.flat_map(fn {{lat, lon}, profile} ->
       build_grid_cache_row(lat, lon, profile, valid_time)
     end)
   end
 
+  defp filter_grid_data_bounds(grid_data, nil), do: grid_data
+
+  defp filter_grid_data_bounds(grid_data, %{"south" => s, "north" => n, "west" => w, "east" => e}) do
+    :maps.filter(fn {lat, lon}, _ -> lat >= s and lat <= n and lon >= w and lon <= e end, grid_data)
+  end
+
   defp build_grid_cache_row(lat, lon, profile, valid_time) do
     temp_c = profile[:surface_temp_c]
 
diff --git a/test/microwaveprop/weather_grid_test.exs b/test/microwaveprop/weather_grid_test.exs
index ec6ebbc1..c8ee2cb1 100644
--- a/test/microwaveprop/weather_grid_test.exs
+++ b/test/microwaveprop/weather_grid_test.exs
@@ -302,6 +302,73 @@ defmodule Microwaveprop.WeatherGridTest do
       assert hd(result).lat == 32.375
     end
 
+    test "with a bounds filter only derives in-bounds points" do
+      # Filter-before-derive: when called with bounds, the builder must
+      # skip the `SoundingParams.derive` work for every point outside
+      # the viewport. Out-of-bounds points are left as raw profile maps
+      # so the heavy per-point derivation cost is bounded by the
+      # viewport size, not the full CONUS grid.
+      now = DateTime.truncate(DateTime.utc_now(), :second)
+
+      profile = [
+        %{"pres" => 1000.0, "tmpc" => 25.0, "dwpc" => 18.0, "hght" => 100.0},
+        %{"pres" => 925.0, "tmpc" => 20.0, "dwpc" => 14.0, "hght" => 800.0},
+        %{"pres" => 850.0, "tmpc" => 15.0, "dwpc" => 10.0, "hght" => 1500.0}
+      ]
+
+      base_profile = %{
+        profile: profile,
+        surface_temp_c: 25.0,
+        surface_dewpoint_c: 18.0,
+        surface_pressure_mb: 1013.0,
+        hpbl_m: 500.0,
+        pwat_mm: 30.0
+      }
+
+      grid_data = %{
+        {32.125, -97.375} => base_profile,
+        {40.000, -75.000} => base_profile,
+        {45.000, -120.000} => base_profile
+      }
+
+      bounds = %{"south" => 32.0, "north" => 33.0, "west" => -98.0, "east" => -97.0}
+      rows = Weather.build_grid_cache_rows(grid_data, now, bounds)
+
+      assert length(rows) == 1
+      assert hd(rows).lat == 32.125
+      assert hd(rows).lon == -97.375
+    end
+
+    test "with nil bounds derives every point (backwards compatible)" do
+      now = DateTime.truncate(DateTime.utc_now(), :second)
+
+      profile = [
+        %{"pres" => 1000.0, "tmpc" => 25.0, "dwpc" => 18.0, "hght" => 100.0},
+        %{"pres" => 925.0, "tmpc" => 20.0, "dwpc" => 14.0, "hght" => 800.0}
+      ]
+
+      grid_data = %{
+        {32.125, -97.375} => %{
+          profile: profile,
+          surface_temp_c: 25.0,
+          surface_dewpoint_c: 18.0,
+          surface_pressure_mb: 1013.0,
+          hpbl_m: 500.0,
+          pwat_mm: 30.0
+        },
+        {40.000, -75.000} => %{
+          profile: profile,
+          surface_temp_c: 22.0,
+          surface_dewpoint_c: 15.0,
+          surface_pressure_mb: 1015.0,
+          hpbl_m: 400.0,
+          pwat_mm: 25.0
+        }
+      }
+
+      assert length(Weather.build_grid_cache_rows(grid_data, now, nil)) == 2
+    end
+
     test "strips raw profile/duct_characteristics/surface_temp_c/surface_dewpoint_c fields" do
       now = DateTime.truncate(DateTime.utc_now(), :second)