The base game can be done in like 5-8 hours on a leisurely pace, the current speedrun WR is 57minutes but there are only 6 runners who've submitted to it so feasibly you could get a lot better times than that.

I've actually got a video on YT where I perform the "I Was Not Expecting You, Human" achievement to Slay Warden Krieg, who is the game's midboss, in Veteran Mode with a character under level 11, and it was an hour and thirteen minutes from character creation to finish. The last 10 minutes are just the Krieg fight itself. This could be done much faster without the level requirement, though, because you could get more damage and better items.

The major problem with it is the enemy scaling. Every area has a minimum level and a maximum level, and as the character levels up so do the enemies. That means if you keep leveling in an area until you dwarf the enemies, it just keeps making it harder in the next area, so you're incentivized to stay at the minimum level for that area and ignore the vast majority of enemies. Even farming for good loot comes at the added cost of making the mobs harder. I used to use a site that shows the level range for areas but I can't find it anymore, sadly.