The weak hash isn't there to speed up hashing. It's there to speed up other things, at the expense of slower hashing.
That's why it says "as it has an acceptable performance impact". That is, using weak hashing has a small performance impact on hashing, but it will bring gains, so it's worth turning it on.
If it had a large impact on hashing performance, then that would outweigh the benefit of the weak hashing, so it would be disabled.