Are you running WSL 1 or WSL 2? Is this emacs in a terminal, or GUI emacs via VcXsrv or similar? Where is the file, on the Windows side or the Linux side? How is emacs configured for, I dunno, swap files or whatever its equivalent is (I’m a Vim man)?
My first guess is that you’re using WSL 1 and accessing Linux files, and that emacs is doing synchronous flushing writes of some kind of swap file on the same file system after every tiny thing. If this is correct, I scream in horror at the magnitude of the wrongness of what emacs must be doing.
My second guess is that you’re running emacs in Windows’ conhost, and it’s emitting vast amounts of data to the terminal for whatever reason. conhost is really slow, though it’s getting a bit better.
If neither of these is correct, I’d be interested to know what is going on. Profiling may well make the answer obvious, though if you’re on WSL 1 I’m not sure if the standard profiling tools will work. If you’re on WSL 1, upgrade to WSL 2, it’s superior and faster in all regards except performance of accessing Windows files, which is way slower (but accessing Linux files is enormously faster). I’m convinced that there will be a simple explanation.