Running a single player flat world, I noticed a constant high CPU load (around 140% of core cycles) from minecraft and, indeed, it couldn't keep up 20 TPS. Using the speed of items through a hopper, I measured it to be running at about 7 TPS. Upon pausing the game, the CPU load drops substantially. I used all minimal video settings including 2-chunk render distance. If I bring up the debug profiler pie chart, it changes very little when pausing the game which leads me to believe it's a server-side issue. FPS remains as good as it has always been.

If it helps, I tested the snapshots dichotomically and found that the issue appears in 14w25b.

Also tried with Java 7 and 8 for same results.