PerformanceModeP should be enabled for production sites, it will speed request processing. The effect isn't huge, but it is worth doing. Do not do this for sites in which you're actively mounting and dismounting packages, unless you're willing to do a server restart after doing so.
PermissionCache is a bad idea used to speed .LRN. I think Sloan may be using it (they wrote it), but I recommend against it unless you really really really need it. It was added back when the permissions system was really slow, now that it's been speeded up the impact of permission caching is not so great. If PermissionCache is set, changes to an individual's permissions or their privilege in regard to an object may not be seen until the cache flushes (defaults to five minutes, I think). Parts of .LRN will flush the cache when permissions change, but underlying OpenACS code for the most part is unaware of the caching ... which was never meant to be a permanent solution.
