[bitfolk] Not being a hog

A bit of help with finding the right command please.

What I'm after doing is a bit of video transcoding on a 4 times a year
basis, I realise this is CPU intensive and would like to be a good
neighbour to the others who are on the same physical box as me.
How do I run a command in low priority mode?

In case the flavour of linux matters I'm on Ubuntu.