[rlpb]

You can attach pv to an existing process. IIRC it's the -p option.

[everybodyknows]

-p is same as "--progess":

https://linux.die.net/man/1/pv

[rlpb]

Ah, it was -d I was thinking of. Anyway, the point is that you can run pv retrospectively just fine while your slow command is running.