I made a pretty insane excel calculator to figure this out for myself. (I am in the process of making a more user friendly version of it to post here one day.)

The TLDR is that I should be putting my mortgage down payments in a TFSA, investing it in something stable and safe (like a GIC, or CASH.TO like I am looking into) and using it all at once when my mortgage renews in 2025.

The interest savings for myself were pretty low, even when I simulated making yearly down payments, but this could be totally different for you. I am already a few years into my mortgage. My rates are low (sub 2%) and the principal remaining is definitely below average.

With higher interest rates or higher principal remaining the answer might change for you.

For myself the answer is even more complicated. I considered not even paying down the mortgage at all and leaving everything in investments as so many recommend, but when I included calculations for my renewal in 2025 and what the bank thinks interest rates will be, it painted an ugly picture.

The increased interest at renewal time actually made making mortgage down payments very valuable. The less principal remaining at renewal time, the smaller the regular payments would be.

The smaller the regular payments, the more free cash flow I had for investments or additional mortgage paydowns. Making principal payments on my mortgage before 2025 had a significant effect that beat out even the most optimistic returns I could get in a TFSA.

(It's also worth mentioning that out of anywhere I can put my money, the most valuable place for me is the spousal RRSP, even more so than the mortgage. Every April I put my down payment TFSA into my spousal RRSP and then fill it back up with the tax return. The only reason I don't put it directly into the RRSP is because it costs me nothing to wait until April and I can still use it if there is an emergency).