Production user research summary

[nikomatsakis]

@aturon

FWIW, I just checked with @alexcrichton and he says it's about 3-5x faster than -O0, which is still much slower than -O3.

OK, yes I misremembered. Here were the results @alexcrichton sent me :

• O0 - 1:01 build time, 1.5s runtime
• O1 - 1:09 build time, 0.5s runtime
• O3 - 2:13 build time, 0.17s runtime

still, much closer to -O3 than -O0.