Grok 4.5 vs Kimi K3for everyday maths

Overpaying early slashes the large starting balance sooner. Interest builds on whatever you still owe, so cutting the debt early prevents decades of interest from accumulating on those extra payments. Near the end the balance is already tiny and little time remains, so the same £100 saves almost nothing.

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Interest is charged on whatever you still owe. Early in a mortgage the balance is huge, so interest makes up most of each payment. An extra £100 then shrinks the balance, and that smaller balance keeps saving you interest every month for twenty-plus years — the savings snowball. Overpay near the end and the balance is already small and mostly principal; your £100 only reduces interest for a few remaining months, so there's barely any interest left to save.