usability - Studies showing the impact of response time variability on user experience

Have there been any studies, or results, focusing on the impact of variability in latency of response times on the overall user experience?

I'm aware of the research on the "generally accepted" recommendations (100ms for instant, progress indicators for 1s+, etc.). However, what I'm curious to know is specifically the perceived performance difference (if any) between cases like:

1. consistent 500ms response time for all interactions


2. average 300ms response time with +/- 200ms variability

Is (1) preferable to (2), vice versa? If either, in all cases? Etc. Of course, 500ms and 300+/-200ms are arbitrary stand-in values.