I recently upgraded my PC to a Skylake i5 (Core i5 6400), which has the new Intel graphics. However, 2D graphics was extremely slow on my 4k monitor. When I wanted to play a small webm, I could be sure Firefox would freeze; switching applications always took half a second. However, switching the acceleration method of the Intel driver from UXA (which was loaded according to
Xorg.0.log) to SNA fixed that.
The following content for the file
/etc/X11/xorg.conf.d/20-intel.conf fixed the issue:
Section "Device" Identifier "Intel Graphics" Driver "intel" Option "DRI" "3" Option "AccelMethod" "sna" EndSection