A revolving card at 20% APR is not "a bit worse" than a 6% loan. It is a compound interest machine pointed the other way: the interest you do not pay is added to the balance and generates more interest, exactly as in a portfolio, but against you.
Hence the practical hierarchy that almost no calculation contradicts: clearing a 20% debt is a guaranteed 20% return, free of tax and free of volatility. No investment offers that. Above roughly 8–10% interest, paying down debt beats investing in nearly any reasonable scenario.
The minimum payment is where the mechanism hides: it is calculated to cover little more than the interest, so that the debt lasts a long time. Paying the minimum is not going slowly, it is not moving.