A long time ago I made an alias always passes -v to git branch and this is the problem: on large repos it can take a long time to compute the "ahead X, behind Y" information. It can be fixed by aliasing to some git branch --format without %(upstream:track)

Those numbers not really useful interactively when very large, for example it doesn't help to print that one of my branches is "behind 132132". Maybe git could print "ahead 7, behind 1000+" for old stale branches stuff? This way it would limit the number of commits examined.