Well, many of us have already faced this problem of browsers acting sluggish and heavy on Linux over time. And then there is the famous memory leak problem with Firefox browser on linux. But with the new Opera 10, things have definitely changed a lot. "Sluggish" is a word not to be found in the Opera dictionary.
Opera 10 also has the Turbo technology which is very useful to users on a low speed internet connection. On enabling this feature, Opera gives up to eight times faster Web surfing over slow connections than other browsers. You might like to read this article on how to enable Turbo for Opera.
PS: Opera 10 is available for almost all Operating Systems out there!