IMO, if both parties are already constantly thinking lustful thoughts and have already smashed faces at least once by the 33% mark, it's not what I personally would call a slow burn even it takes them a while to determine motive and opportunity for the full horizontal tango
Since I assume this is normal novel length, I generally agree; I have read fic I would confidently call slow-burn even though they started kissing a third of the way through, but where "a third of the way through" is after 100k+ words.