All released versions. All official versions. Or, all versions except for 2.6 alpha 1, beta 1, and beta 2. Sorry if I wasn't clear. The only versions of the software that I believe have anything directly to do with the slowdown issue would be 2.6 alpha 1, beta 1, and beta 2. I believe, lacking any other evidence, all other slowdown issues are related to a particular configuration of Internet Explorer and/or Windows. It's also very rare, being reported by at most a few dozen users, out of tens of thousands (at least). Again, lacking any other evidence, I can only attribute it to a particular configuration of IE/Windows. I really, really, REALLY wish I had more to go on. In fact, I wish it was (or IS) a bug in the software. If it were, at least I could have a crack at fixing it.
As I stated in another thread (I think), while trying to figure out the memory issue I introduced in 2.6 alpha 1, I did a lot of optimization. Perhaps that is responsible for the increased performance you see in 2.6.b3. But while helpful, it doesn't solve the issue. I'll keep looking...