For me, IMAP is not slow at all. In fact, I almost never have a problem with GMail over IMAP at all - and I have 3 separate email accounts (some under google apps for your domain) that all check every 30 minutes or so).
Sometimes I have issues with large attachments using Mail.app through IMAP, but I think its mostly Mail.app messing up or google throttling the connection.
Let's not forget that IMAP clients often queue the connection requests, so that when you hit Send it gives them illusion that it was quick, while it's often still sending in the background.
Gmail's web interface would do that too. It's not like it just blocks with a white page while it waits to send.
Would be curious to know if IMAP is slow for Weinberg specifically. Others report good web experience, and disjoint others report good IMAP experience.
It is for me. Loading takes forever. I gave up, only use web now. Web is fast and good for me though.