Oh forget what I said then, it's just usually the problem with RuneMate being laggy. I don't exactly see how ram could be causing lags, the default maximum allowed heap size for the client should be plenty.
To answer your question though, edit the runescape.prm file inside home/jagexcache/jagexlauncher/runescape. Change the line where it says "-Xmx384" to for example -Xmx1g (from 384mb to 1gb).