I see this question pop up a lot in mlm spaces, and honestly, I think we might have different definitions of 'slow-burn' sometimes. Some books people recommend feel more like instant gratification wrapped in a longer plot. A truly painful, delicious slow-burn for me is 'A Marvellous Light' by Freya Marske. It's historical fantasy with magic and bureaucracy, of all things, which sounds dry but creates this incredible pressure cooker for the two leads. The romantic tension is woven through solving a mystery and navigating a hidden magical world, so the actual physical stuff takes its sweet, agonizing time. The payoff is worth every single page.
I'd also throw in 'Wolfsong' by TJ Klune. It's tagged as paranormal romance, but the childhood-friends-to-lovers arc spans years, literally. The pining is off the charts, and the smut, when it arrives, feels like an eruption of all that built-up emotional intensity. It's not just about the wait; it's about the foundation. If a book makes you feel like you've earned the spicy scenes alongside the characters, that's the good stuff.