I'm pretty sure it's the main reason. Look how many times Magit shells out to git to render the status buffer, too many to count: https://github.com/magit/magit/issues/1327
In a perfect world, Emacs would link to libgit2 and Magit would be adapted and folded into the core distribution.
I just called the status buffer from magit on a repo. It was 23 calls to git, mostly for rev-parse. The status buffer appeared instantaneously on a decade-old laptop.
Whatever is causing the slowness you're seeing, I don't think it's just because of the number of calls. It's probably the slowness of one or two specific commands, perhaps due to repo size.
Are you testing on Windows though? I am specifically talking about Windows being the (biggest) problem with this architecture. It is very expensive to create new processes on Windows versus Unix (threads are cheaper on Windows though).
Another commenter said that they also experienced slowdowns on macOS, so I thought the OS actually didn't matter much on this, but based on disgruntledphd2's comment, I guess it does.
In a perfect world, Emacs would link to libgit2 and Magit would be adapted and folded into the core distribution.