The higher the friction, the lower the probability of conversion. E.g. Amazon famously found every 100ms of latency costs them 1% in sales.

At its most simplified, this can be thought of as a simple function of time — the more time something requires, the higher chance something else happens during that time, invalidating the original task.

The best sign-in flow is none at all — that's what e.g. Discord does. They let you use the app immediately, with an automatically created provisional account. Amazing user experience.

This applies universally — convenience is everything.