A single Diablo 4 dungeon is so stuffed with monsters you can gain 40 levels in under 2 hours

pcgamer.com:

Diablo 4 players have discovered a dungeon with enemies packed so tight you can use it to level up from 1 to 40 in around 2 hours.

Patch 1.1.1 dumped 50% more monsters in Diablo 4's Nightmare Dungeons. The way the patch notes are written, it didn't seem like newly created characters could benefit from the buff until they're around level 50 and use an item to turn a normal dungeon into its Nightmare version. But it looks like Blizzard might have buffed both versions. 

The opening section of Domhainne Tunnels (map link), a dungeon located in the middle of Scosglen, is now brimming with monsters in its normal version. I compared videos of players running it from a few weeks ago to how it looks now and found the number of monsters has been substantially increased since the patch. With one specific method, it's now the fastest way to level up in Diablo 4's first season.

Diablo 4 content creator Raxxanterax previously identified Domhainne Tunnels as one of the best Nightmare Dungeons to repeat for XP once you're already around level 50. In a new video, however, he was able to use its opening section to go from level 1 to 40 in a fraction of the time it would take you to finish the first two acts of the campaign.

Here's the strategy for leveling up fast in Domhainne Tunnels:

If you need to sell or salvage items, you can teleport to town instead of leaving the dungeon in step 3, reset it, and then re-enter your portal to run it again.

Sorcerers who use Chain Lightning or Twisting Blades Rogues can easily sweep through the huge packs of enemies quickly. You don't even need to stop running; just periodically pause and kill everything around you until there's nothing left.

Domhainne Tunnels is so fast that Diablo 4 speedrunners