So I recently tried out the Pokemon Sage demo and, good lord, the XP curve was bullshit. Winning the immediate rival battle at the beginning doesn't even get you to level up to 6. The XP gain for way lower level mons was fine but everything else was a complete slog. In addition, there were lots of annoying early level mons with Sand Attack, Defensive Curl or Tickle, making your early physical attacks like tackle less effective. Also mostly ice and normal type mons, so you couldn't take much advantage if you didn't pick the fire starter. And on top of all that the Pokecenter placement was extremely sparse and there was no other healing available so that I constantly had to backtrack. That game also doesn't have a turbo mode implementation yet so...that might have been the worst slog I've encountered, yet. Right from the beginning, you're slowed down to a crawl. Because I was interested in the Fakemon I pushed on nevertheless but eventually resigned some time after the first gym battle when I couldn't handle it anymore. I also only beat the first gym because hidden power was available early on and luckily steel type on one of my strongest mons and thus super effective against the ice type gym with its really high level, powerful mons on the gym leader.
The XP gain curve is something I didn't use to pay much attention, too, but oh boy did I take notice quickly when it's done as badly. Especially at the beginning of the game where you have few options, it's an absolute killer.