Savings rate = (income − expenses) / income. Earn $2,500 and spend $2,000, and you are saving 20%.
It is more powerful than return because it works from both ends at once: raising it increases what you accumulate and reduces the wealth you need, because the target is calculated from your spending. Going from 15% to 30% does not halve the timeline; it cuts it by more.
It is also more controllable. You cannot decide what markets return next year, but you can decide the cost of your housing, your car and your subscriptions. Return is a hope; savings rate is a decision.