diff --git a/OpenRA.Game/Map/Map.cs b/OpenRA.Game/Map/Map.cs
index ef15b56ceb..2f578880cd 100644
--- a/OpenRA.Game/Map/Map.cs
+++ b/OpenRA.Game/Map/Map.cs
@@ -246,6 +246,7 @@ namespace OpenRA
 		[FieldLoader.Ignore] public Lazy<CellLayer<byte>> MapHeight;
 
 		[FieldLoader.Ignore] public CellLayer<byte> CustomTerrain;
+		[FieldLoader.Ignore] CellLayer<short> cachedTerrainIndexes;
 
 		[FieldLoader.Ignore] bool initializedCellProjection;
 		[FieldLoader.Ignore] CellLayer<PPos[]> cellProjection;
@@ -951,9 +952,32 @@ namespace OpenRA
 
 		public byte GetTerrainIndex(CPos cell)
 		{
+			const short InvalidCachedTerrainIndex = -1;
+
+			// Lazily initialize a cache for terrain indexes.
+			if (cachedTerrainIndexes == null)
+			{
+				cachedTerrainIndexes = new CellLayer<short>(this);
+				cachedTerrainIndexes.Clear(InvalidCachedTerrainIndex);
+
+				// Invalidate the entry for a cell if anything could cause the terrain index to change.
+				Action<CPos> invalidateTerrainIndex = c => cachedTerrainIndexes[c] = InvalidCachedTerrainIndex;
+				CustomTerrain.CellEntryChanged += invalidateTerrainIndex;
+				MapTiles.Value.CellEntryChanged += invalidateTerrainIndex;
+			}
+
 			var uv = cell.ToMPos(this);
-			var custom = CustomTerrain[uv];
-			return custom != byte.MaxValue ? custom : cachedTileSet.Value.GetTerrainIndex(MapTiles.Value[uv]);
+			var terrainIndex = cachedTerrainIndexes[uv];
+
+			// Cache terrain indexes per cell on demand.
+			if (terrainIndex == InvalidCachedTerrainIndex)
+			{
+				var custom = CustomTerrain[uv];
+				terrainIndex = cachedTerrainIndexes[uv] =
+					custom != byte.MaxValue ? custom : cachedTileSet.Value.GetTerrainIndex(MapTiles.Value[uv]);
+			}
+
+			return (byte)terrainIndex;
 		}
 
 		public TerrainTypeInfo GetTerrainInfo(CPos cell)
diff --git a/OpenRA.Game/Map/TileSet.cs b/OpenRA.Game/Map/TileSet.cs
index 34896f3d36..d60075d369 100644
--- a/OpenRA.Game/Map/TileSet.cs
+++ b/OpenRA.Game/Map/TileSet.cs
@@ -74,7 +74,7 @@ namespace OpenRA
 		public readonly bool PickAny;
 		public readonly string Category;
 
-		TerrainTileInfo[] tileInfo;
+		readonly TerrainTileInfo[] tileInfo;
 
 		public TerrainTemplateInfo(ushort id, string[] images, int2 size, byte[] tiles)
 		{
@@ -177,7 +177,7 @@ namespace OpenRA
 		public readonly bool IgnoreTileSpriteOffsets;
 
 		[FieldLoader.Ignore]
-		public readonly Dictionary<ushort, TerrainTemplateInfo> Templates = new Dictionary<ushort, TerrainTemplateInfo>();
+		public readonly IReadOnlyDictionary<ushort, TerrainTemplateInfo> Templates;
 
 		[FieldLoader.Ignore]
 		public readonly TerrainTypeInfo[] TerrainInfo;
@@ -217,7 +217,7 @@ namespace OpenRA
 
 			// Templates
 			Templates = yaml["Templates"].ToDictionary().Values
-				.Select(y => new TerrainTemplateInfo(this, y)).ToDictionary(t => t.Id);
+				.Select(y => new TerrainTemplateInfo(this, y)).ToDictionary(t => t.Id).AsReadOnly();
 		}
 
 		public TileSet(string name, string id, string palette, TerrainTypeInfo[] terrainInfo)