[fhars]

Sounds about right. I get up to 11 fps on Ubuntu 64bit on an AMD E-350/Radeon HD 6310 system fullscreen on 1366x768 with the catalyst drivers and hidden GUI and a quick search for benchmarks seems to indicate that it is about twice as fast as the HD 4200 ( http://www.notebookcheck.com/ATI-Radeon-HD-4200.20492.0.html ). You may get a few fps by switching to the proprietary drivers if you do not yet use them, but I wouldn't expect wonders.

[Edit:] And I see up to 13 fps if I fly away from the sun, the author seems to be right when he writes "Calculating sun visibility the geometric way (by casting rays through the terrain and the clouds) can be expensive if not done carefully."