--- a/python/mozbuild/mozbuild/frontend/reader.py
+++ b/python/mozbuild/mozbuild/frontend/reader.py
@@ -1383,23 +1383,37 @@ class BuildReader(object):
             if key not in defaults_cache:
                 defaults_cache[key] = self.test_defaults_for_path(ctxs)
 
             return defaults_cache[key]
 
         r = {}
 
         for path, ctxs in paths.items():
+            # Should be normalized by read_relevant_mozbuilds.
+            assert '\\' not in path
+
             flags = Files(Context())
 
             for ctx in ctxs:
                 if not isinstance(ctx, Files):
                     continue
 
-                relpath = mozpath.relpath(path, ctx.relsrcdir)
+                # read_relevant_mozbuilds() normalizes paths and ensures that
+                # the contexts have paths in the ancestry of the path. When
+                # iterating over tens of thousands of paths, mozpath.relpath()
+                # can be very expensive. So, given our assumptions about paths,
+                # we implement an optimized version.
+                ctx_rel_dir = ctx.relsrcdir
+                if ctx_rel_dir:
+                    assert path.startswith(ctx_rel_dir)
+                    relpath = path[len(ctx_rel_dir) + 1:]
+                else:
+                    relpath = path
+
                 pattern = ctx.pattern
 
                 # Only do wildcard matching if the '*' character is present.
                 # Otherwise, mozpath.match will match directories, which we've
                 # arbitrarily chosen to not allow.
                 if pattern == relpath or \
                         ('*' in pattern and mozpath.match(relpath, pattern)):
                     flags += ctx