I have to agree with this. I have upgraded (or rather it upgraded for me a couple of weeks ago), and now everything I do is delayed by a couple of seconds. Archive an email? I can't close the tab otherwise the browser tells me I'm going to lose data. Send an email? Ditto. I wouldn't mind a better UI, but beside the added lag I don't find there's anything better or different than before.
What happens when you revert to the old Gmail? I'm wondering if you get the speed of the old Gmail back or if it instead just reverts to the non-industrial design elements of the previous version of Gmail. Today a friend told me he reverted all the way back to HTML Gmail because he was so annoyed with the non-responsiveness.
