This is a performance bug that has been in the game since 14w28a. Since I'm not a minecraft modder or computer specialist, the best that I can do is assume what the cause of the bug is.

The "story":
Basically, I'm currently working on a Minecraft map. The map has, in its spawn chunks, a lot of redstone and CommandBlocks (hooked up to a 20Hz-clock) and even though that's a lot to compute, 14w27b handled it completely fine. Starting with 14w28a, the tick speed in the game has dramatically decreased, even though the map was never changed.

Where is the performance decrease noticeable?
I can see this performance decrease on the scoreboard, which has a constant second-timer on it. In 14w27b, it counted up every second, but in the recent snapshot 14w32a it counts up about every two seconds, and basically all the other redstone is slowed down by the same factor as well.

It's the same in 14w32b,c,d but there are some other bugs there as well that I don't want to cover here.

What could cause this?
As I said, I don't know exactly what causes this decrease, but I think that some redundant code which controls chunk updates and redstone/commandblocks has been added.

I think this lag is not supposed to happen, because
1. the redstone is located in the spawn chunks, where any update used to not create any lag.
2. the performance was better in the past.
That's why I think this is a bug, but if it isn't, please let me know.

How to reproduce:
Again, since I don't exactly know what causes this, I'm going to leave a map download so that you can see for yourselves. Please do not play or publish this map in any way, please only do this:

1. Open the map in 14w27b. The timer on the right ("Time") should run in a steady, one-second-period interval.
2. Open the same map in 14w31a. The timer should run considerably slower.
Optional: Check the redstone by going to -1000,64,0.

Map download (only for testing purposes):
http://www.mediafire.com/download/bxjo5ait3c6bz51/SpawnGame+TESTING+MAP.zip