If this is NOT related to compositing you perhaps suffer from the issue 
documented in bugs  178269, 183263, 241094, 243094 - this should (hopefully) 
be fixed in 4.6

If this IS related to compositing (ie. shift+alt+f12 makes everything fast 
again)
* Disable the blur effect
* (esp. when using the nvidia driver) disable decoration animations in oxygen 
and use a not animated aurorae or not aurorae at all (sorry martin ;-)
* blacklist the lanczos filter (see bug #243181)
* in case you've an nvidia GPU, see bug #234463

Thomas
 
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