Kafo

I've personally never seen it that slow, but it certainly feels sluggish as the number of modules has increased. Perhaps it needs some build-time caching? (If the speed can't be improved any other way.)

we could possibly list manifests that contains information printed in help and keep their hashes and regenerate help only if they change

I think we can do similar what Apipie does - find the latest ctime on all the modules (there is no big benefit in fine-selecting only those relevant) and compare to the ctime of the cache file. This should be relatively fast (<1s the first run).