tedster - 6:19 pm on May 26, 2010 (gmt 0) [edited by: tedster at 5:45 pm (utc) on May 28, 2010]
A couple of comments:
1. If you're concerned about the site speed factor causing a massive traffic loss - I'd say relax about that one. First, it's a minor factor. And second, server speed is only the small part of the Speed Speed metric. The bigger part is all about the page's rendering speed in a browser, and those fixes are within your ability to change without moving servers.
2. As for the date, yes there have certainly been ranking changes all along. But in this case we are calling Mayday, we're looking at a roll-out right over the end of April and beginning of May. At the Google I/O conference, Matt Cutts confirmed that it is an algorithm update - something he has very rarely confirmed in the past. < see this article from Vanessa Fox [searchengineland.com] >
So I'd say it's more than a bit of dial twisting around the factors we already know. Although that may also be involved, Matt never called that kind of event an "algorithm update" in the past. Instead it may be the introduction of a new ranking factor[s], or it may even be a whole new methodology.
[edited by: tedster at 5:45 pm (utc) on May 28, 2010]