It is a relatively modern game which uses a lot of resources, even for relatively new systems.
And it is still in beta phase. With start of the Open Beta, the debug code was removed, so now we should have the fastest ShootMania ever since it was first given to test
In fact, the settings have a big impact on the performance.
Please reduce the settings, don't use highest quality everywhere, don't use maximum AA settings, etc.
Make sure your system was set on maximum performance and not balanced.
See if ManiaPlanet.exe is registered as managed game in your video card settings.
Also make sure your video card has enough power available.
And check your have the newest audio and video drivers installed.
Finally, ManiaPlanet is apparently being optimized for nVidia cards, not yet necessarily for AMD cards.