You can look at the compound interest effect as a mathematical thing or a psychological thing.
The mathematical part is pretty straight forward and a bit boring after you understand it (IMO).
I can tell you how the psychological impact hit me. It happened a few different ways and in a few different places.
1. One day I was looking at my brokerage account, and I realized just my taxable brokerage account had nearly $200K of unrealized capital gains sitting there. That's $200K that's been earning me even more money, and I never even realized the scale of it in the day-to-day market movements.
2. I re-did my time-to-FI spreadsheet with a different look and feel. I was playing with numbers and realized that changes to my savings rate no longer had any material impact, and the date was almost entirely a function of my portfolio growth rate assumptions. The difference between saving 50% and 0% is the difference between full FI in 2025 vs. 2026 (Assuming a portfolio growth rate of 4%).
Number 2 is insane. How much did you accumulate to get to that point? I made the same projection, and, by just maxing out 401k+matching (the minimum savings I do in a year), I ended up right at my minimum FIRE number in four years, otherwise it will take seven years of coasting.
To answer OP, over the past year my portfolio has gained 7x the annual amount of maxing my 401k with matching.
My favorite anecdote is from this year in March and April. I spent a month in Mexico, spent way too much (around 6k; more than I've spent on any one item in years), and by the end of the trip I was worth 20k more than I was at the start of the trip thanks to market returns.