Why the early years feel like treading water
Each month the lender charges interest on the whole balance you still owe. Your repayment covers that interest first, and only what's left over reduces the loan. At the start the balance is at its biggest, so the interest is at its biggest, and most of the repayment disappears into it.
As the balance slowly falls, the interest each month falls with it, so more of the same repayment comes off the loan. That's why the chart's lines are curves, not straight lines, and why the last ten years of a mortgage pay down far more debt than the first ten.
What this means for extra repayments
It's the same fact seen from the other side. A dollar you pay off early stops attracting interest for the whole remaining life of the loan. The earlier in the loan an extra repayment lands, the more it saves. The extra repayments calculator shows exactly how much.
What this means for a rate rise
When the rate goes up, the interest part of each repayment goes up straight away. If your repayment doesn't rise by enough to cover it, less of your money comes off the loan and the loan runs longer.