In my custom map, which has about 5 active redstone clocks at the moment, I experience server-side lag and stutters even though I am in Singleplayer. I have not tested this map in a multiplayer map yet. Shortly afterwards, I entered my redstone testing world, set up a rapid 1-tick clock and I still get the server-side lag. This is just one clock by the way.

This might be because of my computer, but I downgraded to snapshot 14w21b, opened up the same map with the 5 rapid 1-tick clocks, and I had absolutely no server-side lag. So I am assuming this is a performance error in the new snapshot.