You can set, how many emails roundcube should load.This is a screenshot from my system, its properly on another lang, then you speak but you can follow the path, and end up the same spot.http://prntscr.com/2k2r0wBtw. its the number 2000 you should change to ex. 500.
Make sure the mailbox isn't being sorted, Read: http://trac.roundcube.net/wiki/Howto_Config/Performance