There are 2 main differences between slow burn and stalling in my eyes especially in romance: 1, believably/contrivance and 2. tension vs teasing.
For example, I read a fantastic story where the two romantic leads had been close as anything, battle partners for many years and went through hell together and very obviously had The Feels only for one to be seriously and life changingly injured. Suddenly they couldn't work together, couldn't communicate like they used to and both had a truck load of issues to work over in the middle of a war. This worked. Every interaction there was clear tension, every interaction meant something or had a step in the right, or sometimes wrong direction, but it always did something. Be it hinting that they can still understand each other they just don't think they can or the very near confession where it's clear that they're actually both afraid that if they say it out loud they'll just lose the other and its easier to stay at arms length after last time. It was amazing and I wish I could remember the name.
On the other hand, you fall into the classic soap opera will they won't they and that get boring so quickly. misunderstandings and contrivances like seeing them talking to someone else ect is a clear sign of stalling to try and add tension and actually I'm just yelling "if you two don't have a good reason for not getting together do you actually want it?!" I can maybe stand that sort of ridiculousness once, twice at a push but if you're going to put me through 50 chapters of "but they were hugging someone else" I'm going to get bored reaaaally quickly.
And yes, it can be more realistic, because love and take a while, and I'm cool with the slow burn variety that is they meet, they move through friends and then to dating and its all taken at a natural pace with clear underlying tension building, also cool.
As with everything else, it's a matter of how well it's done.