Why are Pokémon Diamond and Pearl so slow?

Play Diamond or Pearl today and the slowness is the first thing you feel. The clearest single cause is documented: in Generation III, Surfing moved at running speed, but Diamond and Pearl reduced Surf to walking speed, so every lake and sea route, and Sinnoh has long ones, is crossed at a stroll. Health bars drain in a measured way, move animations take their time, and the whole game breathes more slowly than the Game Boy Advance games before it.

The region itself adds friction. Sinnoh is the cold, mountainous generation: deep snow on the northern routes swallows your steps, mud in the Great Marsh and on the rainy routes pulls your bike and your feet down, and Mount Coronet's caves break the journey into slow, room by room climbs. None of this is a bug. It is a big region rendered at a deliberate pace on 2006 handheld hardware.

Game Freak clearly heard the complaint, because Pokémon Platinum, the 2008 enhanced version, sped Surf back up to running speed and generally tightened the game's pace. Players returning to the originals usually notice the difference within the first sea crossing.

For a collector, the practical point is that the slowness is a property of these two cartridges specifically, not of Generation IV as a whole. Platinum is the same Sinnoh with the governor removed.