stretches or cost is number of hashing rounds, so not related to password length.
bcrypt has 72 bytes password length limit [0]. If the password is shorter, it gets extended to 72 bytes anyways, so longer passwords (10 chars vs 70 chars) are not making time difference.
The password can be pre-hashed to allow arbitrary sized passwords [1], but I don't think 10 chars vs 10000 would make a significant difference. I think most of the time will be spent on bcrypt rounds, not on pre-hashing.
> Happened to me.
While trying to find stretches quote, I found this post [2], so maybe it was not bcrypt? :)