vagabundo
Adventurer
I listen to 3d6 Down the Line podcast and in a recent episode they were fighting a high level undead cleric, a Mummy and a few Shadows. That combat took an hour and a half of the podcast, with one character death. They are in a Mega-dungeon complex and follow the process for B/X (OSE really) dungeon crawling pretty rigorously. Long combats are a feature of D&D. I'm pretty sure my 4e group would have completed the combat in about an hour to be honest.That's not what I am talking about. Those aren't "heavy rules" they are part of the gameplay loop (as @Charlaquin said).
What I mean is a game with 1000 character options and a bunch of situational modifiers and multiple steps to achieving singular and simple results are "rules heavy" and make dungeon crawling unfun. Complex, long, grindy combats. System where you have to count up a bunch of situational modifiers before you can make the roll to do whatever. Basically, modern D&D system isms.
I also remember a guy who had a blog on following the 1e DND combat rules to the letter (as much as thats possible) and the combats took a long time.
You can speed run combats with decent DM planning, but thats at odds with classic dungeon crawling processes. Morale rules do help - which is why I use them in my 4e games.