I feel like the first time having sex is usually just about the worst sex you'll ever have with that person, so it's all but guaranteed to get better. However, I don't think there's a massive shift that happens. It improves with any partner as you repeat it but only by degrees. So if it's miserable the first time, in my experience, you're moving over time to slightly less miserable but still unfulfilling. If it's halfway decent the first time, then cool, we got this. Let's let it burgeon into something beautiful.