FlyFF and Grand Chase aren't very good examples, as their recommended specs are equal to and lower than osu!'s specs listed in the Download section.
Try lowering your resolution to 640x480, turning off all graphical effects, turning Low-End PC on. If it still lags, switch between OpenGL and DirectX to see if that works any better. Close any unnecessary programs you have running besides osu!. You might want to update your graphic drivers as well.
Posting your specs instead of comparing with other games would have been helpful.
If you're running 1.0Ghz and 256/512MB of RAM, your computer might not be strong enough to play.