I was watching for KWin performance while solving that problem: viewtopic.php?f=14&t=98923 (in a few words, effects were slow)
And I noticed some interesting things.

Cover switch effect was smooth only when switching to the same window which had already have focus, while simple window list scrolling was smooth as it should.
Application launcher, krunner, etc. slided smoothly only if all windows were minimized or if one or two windows were open. Generally, all effects were slower (or sometimes even frankly slow) with open windows (proportionally to open windows, which aren't minimized, count). With no open windows (or with many minimized windows) all available effects were smooth.

I'm describing in past tense because I have solved performance problem (as written by link above), and now all effects are smooth for me. But I think, there's something to optimize in KWin to make such effects as cover switch work smooth on weak graphics cards.

I hope this information can help to optimize KWin to make KDE effects even better. Thanks.

This is most likely not related to the actual effect but to the window activation animation of the oxygen decoration
In "kcmshell4 kwindecoration", press configure decoration, then advanced settings, open the animations tab and uncheck "window active state change transitions" - should do.

This is probably due to the indirection and slow pixmap allocation or image/pixmap conversion.
In case it is this: the oxygen decoration will hopefully use the new shadow system for 4.9 where such animations will become much cheaper.

luebking wrote:In "kcmshell4 kwindecoration", press configure decoration, then advanced settings, open the animations tab and uncheck "window active state change transitions" - should do.

Thanks, I've tried that, but it has no effect at all. There's still a big difference when just a couple of windows are active (aren't minimized) and when a lot of windows are active.