I upgraded to the latest Dart Editor (28355) both on Win7-32 and Win8-64 today. On Win7 it appears to run extremely slowly, on Win8 it is fine. Running both the Dart-Editor and the browser concurrently on Win7 I found unworkable. With just the dart-Editor running, it was still very slow to respond to input, but much better (4 v 25 seconds). I did reboot a couple of times in desperation. The Win7 machine is an older dual-core pentium with 4gb Ram running at about 2ghz (I'm on Win8 now). Previously I've had no major problems with the Dart-Editor on Win7 over a period of 6-months or more.
Is this a known problem with 28355?
Do you have a lot of folders in the 'Files' tab?
I find reducing the number of folders or right-clicking and selecting 'Don't Analyse' on some folders helps performance.
I have not experienced anything as dramatically slow as you have reported though.