I feel like RBY have very poor grinding locations overall. It gets a bit better when you get to the Elite Four (and Unknown Dungeon/Cerulean Cave, if you want to grind there), but for a lot of the game, the wild Pokemon give very few experience points. Without being able to rematch trainers, it's slow-going, especially if you want a full team (and even more so if you add a low-leveled team member late in the game). I feel like these games really reward smaller teams (3-4 members for balanced exp gain), unlike the newer ones that encourage you to use as many Pokemon as you want.
I'm only up to Mt. Moon so far on Red (doing a challenge on it, so I only have two Pokemon so far), and it hasn't been too bad. Haven't started the others yet, though, and I figure that my anticipated approach of training everything on Yellow will be way too slow.