If you pay off your mortgage quicker you pay less interest. Not sure what you mean by "fixed". Atleast where I live a standard 25 year mortgage can be paid off

If you pay off your mortgage quicker you pay less interest. Not sure what you mean by "fixed". Atleast where I live a standard 25 year mortgage can be paid off in ~10 if you were to double up your payments, because all of the additional money you're paying comes off the principal. Thus, the non-interest portion of the payment has _more than_ doubled.