Solved my own problem. Should have read the release notes before posting
They've disabled shared memory X pixmaps by default, and apparently this is what kuickshow/imlib uses. They say, "These pixmaps prevent the NVIDIA driver from performing a number of optimizations and degrade performance in many circumstances." Unfortunately, if you use kuickshow, disabling these pixmaps causes a considerable degradation of functionality!
Anyway, it's fixed with:
Option "AllowSHMPixmaps" "true"