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.
* 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.