If I had to pick between the three, I'd say medium is my preferred one. I like it when stories also show the character actually being together, and for that you need the pace to be a bit faster.
I also read a lot of shorter indie games, and I've found I don't mind a faster pace either, if the author manages to sell me on the chemistry between the characters, or there's some kind of established relationship that's well executed.
Slow burn can also be really good when done well, but I've seen a lot of writers confuse slow pace with *no* pace, so I am a bit picky on that one. Plus, the pay off has to be really good to make all the rest worthwhile, and that can be tricky. If the scene when they actually get together falls flat all the rest is ruined too