I just installed RoundCube for the first time (0.1) and I have to agree with the original post: it is completely unusably slow on a large folder (including my Inbox). The suggestion to "archive" messages is not acceptable for two reasons. First, if you archive all the messages to another folder, then that folder becomes slow. So you have solved nothing. Second, no other IMAP email package requires me to keep my folder size small.
My Inbox: Over 10k messages, nearly 200 unread at present. (I've read the subject line, which is sometimes enough for me, thank you.)
Thunderbird: No problem.
SquirrelMail: No problem.
RoundCube: Unusably slow.
All of the above were tested connected to the same IMAP server, which is running on the same computer as SquirrelMail and RoundCube (Thunderbird was run on a separate client system.) Since RoundCube is on the same system as the IMAP server, I tried disabling the database cache of messages, but that had no noticeable effect. The back end email server is also on the same local system (It's QMail Courier IMAP) under Ubuntu. Yes, the hardware is underpowered, but SquirrelMail performs fine.
In addition to the general "load time" performance to display the Inbox, the "check for new messages" feature basically fails to work properly 99% of the time when the Inbox is this big. The little "Checking for new messages" window appears, but when it finishes, the Inbox is not updated with new messages. If I click on the "Check For New Messages" icon, the same thing happens (i.e., the "Checking for new messages" window appears, eventually exits, but the Inbox is not properly updated). With a user with a smaller Inbox, the feature works fine.
RoundCube will not make any serious inroads into the web mail business until it can handle large folders. I know people that have much bigger folders than I.
I saw posts from a year or two ago that someone had modded the code to use server-side sorting to improve performance. Are those changes integrated into the 0.1 release? I will say this: SquirrelMail performance sucks just as bad if you disable server-side sorting (it's configurable in SquirrelMail).