[BuildSystem] Clarify a comment.
diff --git a/lib/BuildSystem/BuildSystem.cpp b/lib/BuildSystem/BuildSystem.cpp
index fe82477..d0811ee 100644
--- a/lib/BuildSystem/BuildSystem.cpp
+++ b/lib/BuildSystem/BuildSystem.cpp
@@ -443,7 +443,11 @@
// output.
//
// We can solve this by caching ourselves but I wonder if it is something
- // the engine should support more naturally.
+ // the engine should support more naturally. In practice, this is unlikely
+ // to be very performance critical in practice because this is only
+ // redundant in the case where we have never built the node before (or need
+ // to rebuild it), and thus the additional stat is only one small part of
+ // the work we need to perform.
auto info = node.getFileInfo(
getBuildSystem(engine).getDelegate().getFileSystem());
if (info.isMissing()) {
