Google didn't immediately reply to a series of follow-up questions, so it's not clear what percentage of people have access to the new version and how many are experiencing problems with it. It's also unclear when Google expects to have everyone on the new version and when the performance problems be solved.
What's clear is that the problems have unleashed a storm of complaints. A search, sorted by date, for "Gmail slow" in the Gmail Help Discussion forum returns about 35 pages of results related to problems with Gmail 2.0.
In addition to the main thread referenced earlier, here is a small sampling of the many others: