I'm not using Nvidia (not rich enough) and I've had this problem with every PC I've used so far, even when using an i5, Intel HD 4000 (i think that's the name of the graphics card) and my laptop's Temp says 40°c. Of course, PC specs help a lot reducing lag, but it's still there, usually dropping 1 frame every slider (even though the fps is still above 200, but I notice it when playing) and making the game unplayable when there are a lot of sliders...
By reading this thread i found out that OpenGL doesn't have that issue. Tested it and it worked, however, i need to use Vsync when using OpenGL because using anything else produces HORRIBLE black and white lines, appearing randomly over the screen. osu! might one day be OpenGL based (peppy said he wants to do that). I have nothing to complain if Combo Fire and Shaders are one day implemented for OpenGL, and if i could disable Vsync without hurting my eyes...