DraStic

I've been playing Pokémon Platinum and although the game was running flawless, at some point today (after playing for a week or two) the fast forward started lagging really heavy.
I never enabled any cheats and tried to make an in-game save and reset it. Nothing worked.
My speed is set up at 300% and the game drops to 100-170 with huge lag.
I got a Huawei P9(not the lite version) 8-core with 4gb ram.

Anyone know how to fix it?

Seems that you're using fast foward constantly, so it's probably throttling. No fix outside of slowing things down or playing on a freezer.