Speed up witness generation

[georgwiese]

Vaguely, we want to increase witness generation by one or two orders of magnitude, until it is no longer the bottleneck in our pipeline.

@chriseth I don't have the best overview of the current next steps, perhaps you could create our link issues here?

[chriseth]

There is this one here: #436

But in general, this is just trying to figure out where we lose time and then fix it right away.