Page loading speed is certainly something to the effect in SERP. But what should be in Webmaster Tools a average load speed so that it would be good? i have average speed 108. Is it fast or slow?
Matt Cutts said it himself, the algorithm change for slow sites only affected less than 1% of websites. They were going after the spammers who crammed a bunch of websites on the same server. http://www.mattcutts.com/blog/site-speed/
Page loading speed is almost only for human visitors. I've heard that after 0.5 seconds loading time you start losing visitors. Just keep it below 2.5 seconds for your users.
I suppose as search engines advance it might be another factor... Since page speed sort of correlates to the quality of the site in a sense
speed varies on the bounce rate of the site, whether the user will stay or not... its also depends on how big the images on your site usually bulky images loads longer...