We used to have a 2nd loan on our mortgage. It was a small amount and our payments were around $139 a month. We always paid more than we owed. We slowly increase the amount over time, until we were up to about $350 a month. All the extra went to principal. Because of the way interest is calculated, our payment due each month was actually less the $139.
It is my understanding that they calculate interested daily based on the amount of the money still owed. So if over time you pay more principal, the less the amount the less interest. It may not be a significant amount but it does add up. Don’t let a mortgage company try to persuad you from paying more (if you want too that is). They would much rather try to convince you NOT to do it. After all they are making a lot of money off you. And for them every penny counts!