In order to update the status buffer,
git has to be run a few dozen
times. That is problematic on Microsoft Windows, because that
operating system is exceptionally slow at starting processes. Sadly
this is an issue that can only be fixed by Microsoft itself, and they
don’t appear to be particularly interested in doing so.
Beside the subprocess issue, there are also other Windows-specific performance issues. Some of these have workarounds. The maintainers of "Git for Windows" try to improve performance on Windows. Always use the latest release in order to benefit from the latest performance tweaks. Magit too tries to work around some Windows-specific issues.
According to some sources, setting the following Git variables can also help.
git config --global core.preloadindex true # default since v2.1 git config --global core.fscache true # default since v2.8 git config --global gc.auto 256
You should also check whether an anti-virus program is affecting performance.