I don't have a problem with claws interface, the problem is that it is very slow with relatively big folders.
The problem with most mail programs is that they're geared to a local store and imap support is mostly an afterthought. The only program really designed around the imap paradigm was mulberry. I'm too used to the thunderbird and claws interface (both are good enough) to like mulberry, but it actually did something right: it just fetches enough data from the server to fill the screen, nothing less, nothing more.
If the screen can show, say, 20 messages, just fetch 20 headers. Scroll down and fetch 20 more. Let the server do the sorting and the threading.
It is fast even on slow speed links, since there's no time lost synchronizing a local cache with the server, and I assume it also helps keeping memory usage low.