For a long, long time, Magit has been super slow for me when I have any minified files in the diff for staging. It was one of those annoyances that I just kind of lived with for a while, but I revisited it today.
Someone else here was having the same problem. They put it down to long lines.
There's a big discussion on that here. It's been an Emacs problem for a while.
For me, the solution from there was the so-long.el package, which is included in Emacs >27.1 (Which I recently updated to - Installing Emacs from source).
to my init file, the problem has pretty much gone.
It's still a teeny bit slow when there's a few minified files in magit status, but way, way better now.