1, Sinnoh was massive. Tons of 2-3 pokemon trainers with trash at or the same level as things in the wild, so you barely got more exp fighting them from the section after the windworks place up until past the breeder npc house. But wait, that's just between gym 2 and 3. And it's like 6 areas, up to 8 if you try heading south from that city with the 5th gym (though I think it's the 3rd in Platinum?). And that entire stretch takes speed runners a solid 20+ minutes skipping most fights, it's more like 2+ hours for most players if they're skipping catching anything along the way.
2, the battle system and bulk. Stall teams that would set up were the meta for all of gen 4. Sweeping set ups could work if you did it early or end game but otherwise a bulk set up could be sent out early and have the staying power to beat half the enemy's team with just one attack move.
make that 3: The engine was terribly bloated/GF doesn't know how to optimize, so every action took twice as long as it should've (seriously, the hp scrolling, move animations, saving, surfing in dp, and at times even walking/running felt like my DS was about to crash )
I mostly battled in battle simulators
But yeah, ohkoing a blissey took an eternity even if they were low level, but one at 100 took so long that the entire universe ended, restarted, ended again and progressed billions more years before they finally got to yellow range. ---