The saving compounds in reverse because each avoided interest charge also stops becoming part of a later balance.
The same overpayment has more leverage when the term is long and the remaining rate is high.
A fee or penalty can erase the saving, so compare the contractual settlement figure rather than only the schedule.
The full explanation
Compound interest and loan repayment look like separate topics and are the same arithmetic viewed from opposite ends. Once you can see a loan as a savings balance running backwards, the things that surprise people about mortgages — why the first years barely touch the principal, why a small rate change moves the total so much — stop being surprising. Read the complete guide for the reasoning, examples and definitions behind this answer.