Investment success might make some difference in how quickly you can retire, but the various planning around early retirement makes conservative assumptions about long-term returns. The fraction of your income you save makes a
far larger difference than how much index funds gained or lost in any given year.
This holds true not only because the money you save goes to fund your future retirement, but also because your retirement income doesn't need to match your current income, only your current spending. So if you save 40% of your income, you need your retirement savings to produce 60% of your income; if you save 70% of your income, you only need your retirement savings to produce 30% of your income. That means you both increase your savings rate and decrease how much savings you need before retirement.