GMail + Email Client

Hi there! We have moved to GMail from our own servers. For good reasons I have 4 mailboxes that I use. (At least one of which receives 200 messages per hour with a rule in place to move some of the content into folders). I am using Thunderbird right now and the performance of GMail vs our Dovecot (Linux IMAP server) is sluggish. In addition mail takes longer to arrive and polling seems to cache old message lists. Given that we cannot move away from GMail is it a client issue? Is there a better client? Any feedback welcomed. Have a great day. P