I really appreciate your taking the time to reply so extensively. However, the problem definitely lies somewhere else. I'm actually surprised I'm the only one to encounter it, apparently, because my installation is pretty clean & straightforward.
I used many distros and drivers (arch64, arch32, ubuntu64, ubuntu32, chakra, etc.) in the past and never encountered such a dramatic speed decrease.
* The driver is not responsible, because :
- other apps are fast (HD video, 3D apps, webkit browsers)
- the result is the same with the OSS driver and with the proprietary one with compiz enabled (which is itself very smooth)
- I remember it was even faster with a vesa driver
* 32 bits plugins are not responsible because maps.google.com doesn't use any plugin (and I don't think I installed any 32 bits plugin. Maybe the installer installs 32 bits flash by default, but there was no flash applet anyway)
* Memory is not an issue (4 GB here, and very little memory is actually used, no swapping etc.)
I think it has something to do with cairo, or something like that. I have no clue though, I'm just an user