Harry Roberts on how to set a Performance Budgets if you really don’t have a clue where to start:
Time and again I hear clients discussing their performance budgets in terms of goals: “We’re aiming toward a budget of 250KB uncompressed JavaScript; we hope to be interactive in 2.75s”. While it’s absolutely vital that these goals exist and are actively worked toward, this is not part of your budgeting. Your budgeting is actually far, far simpler:
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.