Roughly 40% better compilation speed. Nice. Performance numbers from the bottom of the CL:

All benchmarks are from my 8 core 2.9 GHz Intel Core i7 darwin/amd64 laptop.

First, going from tip to this CL with c=1 costs about 3% CPU and has almost no memory impact.

Comparing this CL to itself, from c=1 to c=2 improves real times 20-30%, costs 5-10% more CPU time, and adds about 2% alloc.

From c=1 to c=4, real time is down ~40%, CPU usage up 10-20%, alloc up ~5%

Going beyond c=4 on my machine tends to increase CPU time and allocs without impacting real time.