Many forums say, that enabling Increase Contrast option in Accessibility Preferences helps Yosemite to make the animations, like entering Mission Control to be much smoother. This is also consistent with my experience.
Increase Contrast off: https://www.youtube.com/watch?v=ctOj1U4OZio&index=3&list=PLsZWReFinrU5GLvs7ILwGvAVTICcjLhTD
What I am interested in, debugging what causes the general slowness of Yosemite interface. It is clear, that at least one of the reasons is canceled when I turn Increase Contrast on. Common sense says, that Increase Contrast turns on Reduce Transparency option too, so it might be the reason, because calculating the transparency of the semi-transparent layers is a hard thing to do. This is not the case, because enabling Reduce Transparency only has no effect on the speed at all.
So the question is: what does Increase Contrast switch? Some GPU parameters? Less textures? Using other rendering algorithms? Something has to be.