Since upgrading to Yosemite, why is the screensaver unresponsive for a few random seconds after it starts? Since I've upgraded to 10.10 (from 10.8), the screensaver is now unresponsive for some random seconds (~1 to 10) after it starts.

This is very annoying, since I like to have a short "screensaver: start after" time, coupled with a "security: require password after", so that the screensaver starts, but I immediatly hit a button and I can get back to work.

I've already had to increase the "require password after" from "5 seconds" to "1 minute", but this doesn't save me from wasting those unresponsive seconds each time...
It likely was screensaver's fault: I was using "Word of the day", it hasn't happened any more since I've switched to "Classic".