Since the moment you enter in your map (single player) you have some moments until all the game freezes, ALTHOUGH you can move, place and destroy blocks perfectly and smoothly, you are no able of running commands, picking up objetcs or throw snowballs and arrows. Chunks that wasn't loaded before the freezing won't load. Water doesn't flow in new directions and all redstone is frozen.
In fact, if the game freezes while an arrow is in the air, it won't fall anymore.

If you exit the map and restart it, the lag is gone for a minute or so.

I don't know if this happens in survival because I can't run any command.

Edit: The amount of computer power when it happens is in normal values between 10% and 30%.

This never happened to me in any version until this one.

2nd Edit: I can easily reproduce this bug using a redstone block clock running some commands. I include the development console in a txt document.

3rd Edit: It seems to be some kind of world corruption, because if I play the map where I found the bug in 14w07a it happens the same. In fact, I think that the first time I found the bug was after playing in 14w08a, then in 07a and finally in 08a. Because of this I think that 14w08a currupts the worlds if you go back to 14w07a (I don't know if it is only with 14w07a or others too) and then play them in 14w08a.