Slow is smooth and smooth is fast. Going slow (as long as it is deliberate/well thought out) will reduce errors or omissions and lead to better outcomes. This means that you can often accomplish what you set out to do the first time. But if you move too quickly, more often you’ll miss the little details that lead to your greatest success.
Sometimes, you just have to act without thought. That can be useful and necessary. But for most situations, it’s the pre-action decisions you make that lead to you achieving your goals.