Paying back a loan quickly will not lower your credit score. If you have a line of credit that closes as soon as you pay off a loan (eg a car loan) your score can go down if it changes your utilization rate (how much you could take out in loans vs. how much you have taken out) but paying off a loan early won’t impact your score.

You can look up what things are factored into a consumer credit score. You can see for yourself that early payments are not part of the formula.