Claude Opus 4.8 vs Grok 4.5for everyday maths

Early in the mortgage, you owe the most money, so interest piles up fast on that large balance. Overpaying now shrinks that balance immediately, meaning every future month's interest is calculated on a smaller amount. That saving compounds year after year for the rest of the term.

In the final years, you owe very little, so there's barely any interest left to save, and hardly any time for the benefit to build up. Timing matters because interest feeds on both size and duration.

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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