Could you refinance for a 15 yr term and get an even lower APR?
Paying more than your minimum payment each month will reduce Principal faster as well.
I'm guessing your index fund is making you 7-10% per year? so you are giving up 2,5% to 6.5% each year if you pulled the money out of the market for that payment. If you anticipate a drop in stock values then diversify your savings with stocks, bonds and principal guaranteed savings.
Paying more than your minimum payment each month will reduce Principal faster as well.
I'm guessing your index fund is making you 7-10% per year? so you are giving up 2,5% to 6.5% each year if you pulled the money out of the market for that payment. If you anticipate a drop in stock values then diversify your savings with stocks, bonds and principal guaranteed savings.