You need the fine teeth to get enough friction. Set up a guide so you
can go as fast as possible, just shove it through. 3" to 6" per SECOND
is not unusual! It will leave a nasty burr, the blade does not cut at
all, it just wipes the metal away. Figure out some nice way to deal with
the burr, die grinder or disk sander held at 45 degrees. You noted other
comments on eye and ear protection, this is a serious menace to both!

Jordan wrote:
Today I tried a coarse-pitched blade running backwards (haven't got a
fine one yet). Result is similar to using an abrasive disc and angle
grinder.
With circular saw, it gets a straighter cut though.

Jordan