I need to cut a 3" cast iron vent pipe to tie to a new PVC vent.

I am using a Milwaukee sawzall with a metal blade and it kept skidding and
wouldn't start the cut at the right place. I then used a 4-1/2" angle
grinder with a metal blade and cut a little notch and now I can engage with
the sawzall without skidding.

However after 15 minutes of cut I don't think I am making a significant dent
at all.

I used the same sawzall and cut through copper and EMT electrical conduits
like knife through butter. But this CI pipe I am not making any progress.

I can't use the angle grinder there is simply not enough room around it.

I went to HD and they recommended a "grit iron" blade. Got that and it
seems to cut a little bit, but again after 15 minutes, I am not even 10%
through the pipe. The way I am going it would take 10 hours.

I remember seeing another blade named "ugly metal blade" I wonder if that
would be better.

Is there a better way to do this?

I know a chain cutter may be used, but I want to avoid it. Part of the
reason I am cutting this is because the lower section was cut by a plumber
who used a chain cutter but created a vertical crack line and now I am
trying to cut away all the damaged section.

Thanks,

MC