In one of the repos I'm working on now we have the setting
"base": "git:branch-unique(origin/master), git:merge-base(origin/master), arc:prompt"
in our .arcconfig. I've discovered the hard way that git:branch-unique is unbearably slow - it runs one command like git -c column.ui=never -c color.ui=never branch --contains 26d83a1aa5cb2ce166ede5572736bd37b per each diff on my current branch when I arc diff, and each of these commands takes 2.2-2.5 seconds for me, which adds up quickly - on a branch with 7 commits, I recently saw it take about 16 seconds on this phase of arc diff (the whole commmand took just over 60 seconds - the other big problem we have is slow linters). It might be worth mentioning that this repo has ~200 branches, most of which are irrelevant pretty much all the time, but might be part of the reason this git branch command is so slow.
We are likely to change our default to get:merge-base instead of git:branch-unique, partly due to the performance problem - and partly because git:branch-unique has some pretty big rough edges for the workflows it intends to enable.