Before 'major' 0.6.5 we improved Web Optimizer a bit with the following things.

Added cache re-generation instead of cache clean up. Now all files are in cache cleaned up, then Web Optimizer tries to re-generate them. While cache files are being generated website doesn't use Web Optimizer - to prevent CPU overload with clean cache. Only after we have all files ready to be served, we can completely activate Web Optimizer
Added 5 cache file sets generation on options' save. The approach described above can be used when options are saved.