Claude Opus 4.6 vs Kimi K3for everyday maths

**It's all about time.**

Every £100 you overpay early on stops interest being charged on that £100 for the remaining 20-plus years. Since mortgage interest compounds — meaning you're charged interest on interest — removing even a small amount early creates a snowball effect of savings that grows over decades.

Overpaying near the end only eliminates interest for the short time left, so there's far less compounding to avoid. In short, early overpayments have more *time* to work in your favour, and time is the most powerful factor.

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.