3 ms·

You can notice sub-200ms lag, but for when you aren't specifically watching for lag or firing a series of rapid events one-after-another, sub-200ms seems to be

You can notice sub-200ms lag, but for when you aren't specifically watching for lag or firing a series of rapid events one-after-another, sub-200ms seems to be roughly the point where it "feels instantaneous."

Splines 9y ago
In my experience a little under 100ms "feels" instantaneous.