I tried couple of times to workaround the problem you're describing through PCH on some of my ex projects. All of them were on the MM LoC scale and build-from-scratch was taking roughly 45min to 1hr on a developer machine. I never got to make a substantial cut in the build time through PCH, only perhaps by a very few % but YMMV.

Given that modules are PCH in disguise, I don't believe modules are going to have a dramatically bigger, and better, effect on C++ build times. Whether or not they are going to cut the build time from 45 seconds to 30 seconds I also couldn't care less - small projects don't even have the build time problem worth solving. Mid and big projects is where it will really count and if modules are going to have a substantial effect in the range of at least 20-30% in build time cut, I will be all ears.