About a year ago, we committed the patch into add a variable "features_rebuild_on_flush" that could be used to bypass the rebuilding of features whenever cache is cleared. This option is great for speeding up some sites, but has the following bad side-effects:
1) Installing a Module or App that includes a Features Override does not automatically apply the override to Features that are currently in their default state, forcing a manual "features revert"
2) Install profiles need an additional feature_revert_all before a site can be used
3) Changing the code to a Feature and then clearing cache does not properly rebuild the feature.
There might be some other cases that don't work. But most of the time it *does* work and it saves a significant amount of time during cache clear. On a typical Open Atrium site that heavily uses Features and Features overrides, the rebuild can more than double the cache clear time (15 seconds extra on my dev machine).
*Most* of the time, nothing has changed on the site. Poormanscron runs periodically and flushes the cache, causing this extra feature rebuild delay just to determine that nothing needed to be rebuilt. So it would be nice to enable this "features_rebuild_on_flush" variable if the above issues can be addressed.
I'll submit a patch that at least addresses issues 1 & 2 above shortly.