This bug will depend on IMAP bugs. Most of these bugs are pretty similar and 
could be summarized as "please rewrite imap handling". 

Current implementation seem to handle it the slowest possible [or maybe not, but 
looks suboptimal]; re-reading whole folders multiple times,  opening-closing-
expunging on every filtered message, and likes. (For an inbox with 1000 msgs 
filtering didn't finish in 60 minutes.) GUI "freezes" when reading, updating, 
filtering, moving from IMAP folders (this can take minutes or hours, depending; 
and I don't mean socket T/O). 

To filter incoming inbox automagically is currently not possible (apart from 
repeatedly freezing SC for hours). I had to turn off filtering-on-receiving(#1), 
and I cannot filter manually either(#2), have to move mail manually to a local 
folder (#3, or by using processing rule but that's very inconvenient because 
only runs when re-selecting folder, which triggers an imap re-read automatically 
which takes again minutes), and filter from there. (As far as I see this cannot 
be automatized right now, so automagically fetch imap to local folder and filter 
from there, correct me if I'm wrong. It is not a real solution, since it empties 
the inbox and often it is not desirable since people want to keep their mail in 
the imap boxes to be able to read it from anywhere, but [would be] better than 
nothing.)