I have to ask... why the extra lettuce? At In-N-Out I sometimes ask for less lettuce because I feel like it's unnecessary filler compared to the other ingredients, lol. Maybe they've just been giving my unused lettuce to you...
Lettuce is one of the least appealing things I can imagine adding to a half decent burger. I’ve also never once cooked a burger I’d describe as “slimy”.
Obviously it’s all subjective at the end of the day.