zxcvbn is the most widely used password strength estimator with 1M npm downloads a week. It's also 389KB gzipped and hasn't shipped a commit since 2017. Most sign-up forms are hauling that around just to block password123.
Poor password UX is a real conversion problem. A strength meter that adds 389KB to your bundle delays page load — on mobile, measurably so. Users who hit a slow registration page don't wait. They leave. The irony is that most of that weight goes toward catching passwords nobody is actually using to register on your site.
So I built passcore - 3.0KB gzipped and 98.4% detection rate on real breach data - same as zxcvbn, benchmarked against a deduped list of passwords pulled live from RockYou, Adobe, HIBP, and other major leak lists.
zxcvbn takes ~9.7ms to load — it's parsing 389KB of dictionary into memory on every cold start. passcore loads in ~0.2ms. It evaluates a password in ~2,600 nanoseconds. For a registration form, it's effectively invisible — no jank, no layout shift, no contribution to your Core Web Vitals score. The strength meter shows up before the user finishes typing their first character.
How it works:
