Even with "Reduce Transparency" enabled (blur disabled) I've had times when Mission Control operates at 2 frames per second on my 13" 2013 rMBP. This never happened using Mavericks. If you try full screen zoom on Yosemite with an external display attached, it's similarly unusable.
I found if you enable the "Increase Contrast" accessibility feature, performance is improved, but it does negatively affect aesthetics.
I answered my own question about this on Apple StackExchange here: