Not a slog these days, maybe slightly on the first play through. Find a good starter build, there's lots of very good builds to start out that wreck pretty much everything until much later.
There's also special items that help with levelling you can find in the game.
It's still a slog. Your perspective is askew. I don't think PoE is a slog, but ask people new to the game. For both games, expansions have made it a massive slog.
In Grim Dawn I made it to the end of regular difficulty, through one expansion and part of way through another and ran out of steam, with an easy, fun, and accessible build I looked up. I then just decided to go to the next difficulty and start the regular campaign again but was already burned out.
I think in a lot of cases if you are invested in a game from the start you don't notice additional slog from expansions and added content because you immediately know what to skip, what is important, what is useless, etc. A player new to the game wants to, or thinks they have to, do it all. And they tend to burn out.
Yeah, you can watch tutorials and videos and what not but having to do that to get through a game just adds to the slog, you know?
It is a tough tightrope to walk to be sure.
Edit - Titan Quest has a similar problem. I played that game to the very end on base, then the expansion made the game BETTER but that extra act really added to the time and challenge at higher difficulties (Nightmare Hades was a bitch). Then they added a large expansion and when I went back to play through it again I got burned out 3/4 of the way through that new content and said fuck it. And they recently added another in Atlantis. I guess my conclusion here is that one expansion to improve on the base game and add a little more content seems like a natural limit, and more than that tends to push people away I think.