Harry Roberts on how to set a Performance Budgets if you really don’t have a clue where to start:
Our budget for [metric] is never-worse-than-it-is-right-now.
Harry suggest to measure in periods of two weeks (or whatever the length of your sprints I guess) and always compare against the previous value. If performance is equal or better: great, you’ve got your next maximum to compare against next time. If performance is worse: you’ve got work (or some serious explaining) to do.
By constantly revisiting and redefining budgets in two-weekly snapshots, we’re able to make slow, steady, and incremental improvements.