Why does Jest --runInBand speed up tests?

Reading your linked page and some other related sources (like this github issue) some users have found that:

...using the --runInBand helps in an environment with limited resources.

and

... --runInBand took our tests from >1.5 hours (actually I don't know how long because Jenkins timed out at 1.5 hours) to around 4 minutes. (Note: we have really poor resources for our build server)

As we can see, those users had improvements in their performances on their machines even though they had limited resources on them. If we read what does the --runInBand flag does from the docs it says:

Alias: -i. Run all tests serially in the current process, rather than creating a worker pool of child processes that run tests. This can be useful for debugging.

Therefore, taking into consideration these comments and the docs, I believe the improvement in performance is due to the fact that now the process runs in a single thread. This greatly helps a limited-resource-computer because it does not have to spend memory and time dealing and handling multiple threads in a thread pool, a task that could prove to be too expensive for its limited resources.

However, I believe this is the case only if the machine you are using also has limited resources. If you used a more "powerful" machine (i.e.: several cores, decent RAM, SSD, etc.) using multiple threads probably will be better than running a single one.

When you run tests in multi-threads, jest creates a cache for every thread. When you run with --runInBand jest uses one cache storage for all tests.

I found it after runs 20 identical tests files, first with key --runInBand, a first test takes 25 seconds and next identical tests take 2-3s each.

When I run tests without --runInBand key, each identical test file executes in 25 seconds.