It sounds like the fsync bug on ext3, that was "fixed" in rc2. (For me, things aren't quite
as smooth as in FF2 but it's a big improvement on previous versions of FF3.) But it shouldn't
manifest itself on images as far as I know. It is most obvious when typing in the address bar
and on some other operations involving sqlite.
The only image-related thing I can think of is colour management support, which apparently
causes a performance hit, but for that reason it is disabled by default and can only be
enabled via about:config. Perhaps slackware enables it by default.
Google for links.