If you have good files it should be quick. The original way miters were cut was with aircraft shears you can waste a lot of the miter that way, and then there is only a small amount to file off, but the guys who use files on bike tubing don't seem to bother.

around 2:00

https://www.youtube.com/watch?v=KIxCdTRkRHo

You can do the hacksaw bit with shears, on the kind of tubing we use. Not the BB or head tube, maybe, but then they receive miters.