Four test videos were used: Three from the Xiph.org test-video library, and the opening title of Friendship is Magic as an animation comparison. Each of these was encoded four times: Once at 'standard' x264 defaults, one at the same with the addition of no-fast-pskip, once at 'extreme' settings as might be used when trying to encode for very low bitrates and once at the same settings with the addition of no-fast-pskip. Overall, sixteen encodes were performed: Four videos, each under four x264 settings. All encodes were performed under Scientific Linux using mencoder SVN-r34833-snapshot-4.4.5 and x264 core 122.
All but one of the videos both an increase in bitrate and a lowering of SSIM when no-fast-pskip was added. The only video in which SSIM increased with the addition of no-fast-pskip was the Rush Hour video, when the slight increase can be dismissed as a consequence of the increased bitrate. After these tests, I conclude that no-fast-pskip is of little practical use.