I started a new 4 camera project this morning and preview performance was terrible. I realized I had turned on GPU acceleration yesterday. I turned it off, and now it's all good.

Not saying the feature is bad, but for some folks it is clearly better to turn it off.

I found the same,
Cineform 1080p x 6 tracks-
GPU on = 15-17 fps at Preview Auto,
GPU off = 25fps at Preview Half.

Might be a Cineform thing, that's what I'm editing also.