After I upgraded my guest OS from Mavericks to Yosemite, the guest os display refresh rate becomes very slow, only 8-10Hz. My host OS is already Yosemite before that. So that seems not caused by host OS. I changed many settings: adjust memory from 2GB to 4GB, display memory from 128MB to 512MB, or use "reduce transparency" in guest os, but the problem still exists.
