Added comments in performance sensitive code.

OpenRA.Mods.Common/Pathfinder/PathSearch.cs

@@ -19,6 +19,8 @@ namespace OpenRA.Mods.Common.Pathfinder
 {
 	public sealed class PathSearch : BasePathSearch
 	{
+		// PERF: Maintain a pool of layers used for paths searches for each world. These searches are performed often
+		// so we wish to avoid the high cost of initializing a new search space every time by reusing the old ones.
 		static readonly ConditionalWeakTable<World, CellInfoLayerPool> LayerPoolTable = new ConditionalWeakTable<World, CellInfoLayerPool>();
 		static readonly ConditionalWeakTable<World, CellInfoLayerPool>.CreateValueCallback CreateLayerPool = world => new CellInfoLayerPool(world.Map);