There is no issues with roundcube on our servers. It sounds like a slow network (server or your client's end) or a flaky installation. I would guess a slow network rather than anything else from either end.
It's slow for me personally even. I am on a 20 megabit connection with plenty of computer behind that. The original roundcube and everything else off that server delivers fine, but this version seems slow. I did a fresh install of it in its own directory. I think it was 3 stable that I installed?