-* Issue #154: Cache the reading of the zip file index for cases where the
- same zip-file is used for multiple packages (like PEX).
+* Issue #154: ``pkg_resources`` will now cache the zip manifests rather than
+ re-processing the same file from disk multiple times, but only if the
+ environment variable ``PKG_RESOURCES_CACHE_ZIP_MANIFESTS`` is set. Clients
+ that package many modules in the same zip file will see some improvement
+ in startup time by enabling this feature. This feature is not enabled by
+ default because it causes a substantial increase in memory usage.