This is kind of a long and rambling post, but I hope it makes some sense.
First off, a data point that caused this whole ramble. The following graph comes from 100 runs of Yosys and nextpnr on picorv32_large.v from yosys-tests, targeting an iCE40HX8K.
![foo](https://camo.githubusercontent.com/9e4d2cd02e164e5e9b52ce111875e24e4e81736392e5e4b01ecbfa63b91f7fdd/68747470733a2f2f7075752e73682f4639786b722f343236623665373364362e706e67)
Obviously, ABC1 resulting in a higher average Fmax than ABC9 is concerning, given that the point of ABC9 is to improve synthesis quality compared to ABC1.
But this got me wondering: how do we actually quantify that a change produces a better maximum frequency?