The idea of bisect is that you only need to do log_2(n) steps to find a bug in "n" commits. Even if you have to do manual testing, it's still normally pretty fast, unless you have hundreds and hundreds of commits.
You're not considering the time it takes to manually test.
* How much time does it take to do a clean build?
* How much time does it take to do a deploy?
* How much time does it take to reproduce the issue manually?
Add those together and multiply them by the number of commits bisect makes you check. It can add up.
Absolutely, and I'm not claiming that bisect is useful for debugging every regression. But it's saved me a LOT of time on a number of occasions, and shouldn't be dismissed out of hand just because it relies on manual testing. Even if going through 6 or 7 bisect revisions takes a few hours, it can sometimes be completely worth it.
