As many folks have noticed, gmail can be really slow with claws-mail. I did not have this problem with thunderbird. I found this article:
That may be a work around to the problem. I have no idea of claws-mail uses the CONDSTORE option, but if it does, could you put in a way to disable it as that may fix the gmail slowness. For me it would be worthwhile as I only have one client writing to the mail folders at a time.
Claws Mail doesn't make use of that IMAP extension.