Well, not necessarily. I wanted to test that theory a while ago, so I performed a little experiment.
I remembered how fast my Hero was when I first bought it, and wanted to see if I could bring that back. So wrote down all the apps I had downloaded, back up all my contacts, music files, pictures, bookmarks, etc. Then I performed a factory reset.
Of course, it was blazing fast after the factory reset. HOWEVER, as soon as I restored all my backed up data and redownloaded all my apps (which took about 10 minutes) my Hero was slow again. In other words, 10 minutes after a factory reset, my Hero was slow again.
So its not the wipe itself that speeds the phone necessarily, but more like the absence of data.
But when I asked the original question, I was addressing those who had restored all their backed up data, asking if the ROM was fast even AFTER all apps were redownloaded, contacts resynced, music and pictuers reloaded, etc.
Can anyone follow up on that?