My ISP recently switched to roundcube for their webmail. Looks pretty slick, though I'm not really convinced it's as good as it should be. It's slow to fetch my email, and searching for specific emails doesn't work very well.
Yeah, there can be a couple causes for this. Sometimes it's the skin being used. Some Roundcube skins are quite a bit faster than the default; you might check with your ISP to see if they support additional Roundcube skins. (They are very easy to install, and users can select an RC skin from their settings panel.)
> ...and searching for specific emails doesn't work very well.
There should be a tiny dropdown arrow next to the magnifying glass icon in the search box. (This is not great UI.) Click that, and you should get a menu like the following:
Subject, From, To, Cc, Bcc, Body, Entire Message.
Make sure Entire Message is selected. It's not by default. RC will remember your setting for each folder. This will make your searches slow on folders with lots of messages, but it improves the search results quite a lot.