Claude Opus 4.6 vs GLM 5.2for 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.

Mortgage interest is charged on the amount you owe. Early in a 25-year mortgage, your debt is at its highest, so you are paying the most interest. Overpaying £100 a month at the start immediately shrinks your debt. This means you are charged less interest every single month for the next 20-plus years. In contrast, making those same overpayments in the final five years only saves interest on a much smaller balance for a short time, missing out on decades of savings.