Server chunk ticking is extremely slow, especially when loading new chunks. This slow chunk ticking (around 1-3 ticks per second) occurs even with normal exploration. When in singleplayer, this can cause the "ticking" to use an abnormally large amount of CPU.

Seed 1, render distance 8, travel from spawn to x=2000, v1.7.2: 19.94 ticks/sec
Seed 1, render distance 8, travel from spawn to x=2000, v1.7.4: 11.03 ticks/sec

I suspect it is a side-effect of MC-12964 , in part, as well as the recent changes made to the rendering and network code in 1.7.
This is a Minecraft forum thread discussing the same effects: http://www.minecraftforum.net/topic/2218534-graphics-quality-and-performance-in-164-172-and-174/

Related: MC-43395