It appears to be a known Firefox "bug":
https://bugzilla.mozilla.org/show_bug.cgi?id=657603 (and a few others mentioned on there)
http://stackoverflow.com/questions/4022804/why-scrolling-with-moz-radial-gradient-is-so-slow
I'd say we should only change something if the lag is significant. Then we probably use a simple SVG as a background image instead? (That way, the colours are still easy to change.) So, how bad is the lag? (I cannot properly test it at the moment.)