This issue was initially about Changes-window in the bottom of the main window.
Please also add similar feature to stage/unstage certain hunks in a separate Changes-window opened by right click on a file - "Show changes" menu item in all modes: (Index vs Working Tree), (HEAD vs Index).

I recommend to send bug report to Git: https://github.com/git/git

They receive it using mailing list: git@vger.kernel.org

Multithreading today is necessity due to new processors with many cores and constantly increasing workload.
Today we cannot allow intensive CPU-related work to use only 1 worker thread when we have 32 available (for example).

I also want to explain why should I ask for that.
My overall goal is to move through changes in "Changes" window and make use of keyboard hotkeys to stage/unstage (or revert) the next change (instead of clicking small buttons "Stage hunk", "Revert hunk" in a splitter).

Explain what do you mean?