I don't mind a romantic relationship sparking up quickly, so long as the author knows what they're doing regarding the growth of that relationship after it's established. That's where a lot of younger authors fall flat, since they've never actually been in a long-term relationship. So they tend to fall back on toxic behaviours, miscommunication, and petty squabbles in order to keep the relationship 'interesting' to readers.
Many IRL relationships spark up quickly. If you have two adults who know they're interested in each other, it won't take very long for one to make that plain, and invite the other on a date or something. The will-they-won't-they stuff is fun for a while, but I find lingering there too long is rather unrealistic for a lot of fictional relationships involving adults.
But the evolution of a budding relationship is endlessly interesting, and far less explored in popular fiction! I enjoy reading about how two characters move from infatuation, to the early stages of love, to that deeper point of love where your partner doesn't give you butterflies anymore, but rather a soul-deep comfort. What happens when two characters are moving through these stages at a different pace? How do they go about communicating that? When is it the right time to establish if the relationship is serious, or has the potential to be? When is it appropriate to admit that you're falling in love?
It's hard to find stories which deal with the above, since most stories tend to have their romantic pairs get together close to the end, and we never get to explore what happens afterwards. This is certainly the case in most YA fiction. One notable subversion of this is The Dragon Prince, and I'm really, really excited to see how they handle their main ship in upcoming seasons. (Though not without some trepidation. Teenage characters mean teenage miscommunications, and I hope they don't lead too hard into that.)
I have rather forthright characters, for the most part, so most of the romances in my stories don't take very long to begin. There'll be one notable exception in Dragonfeathers, but that's because one character is actively grieving the death of his lover, and while he and his love interest both recognise they're falling for each other, they also both understand he's in no right place to be jumping into a new relationship just yet. That one is proper slow-burn, because they both flirt with each other, they know what's going on, but they choose not to cross that threshold until he feels ready to instigate it.