Minecraft uses CPU when u first boot it up and resorts to GPU later

I noticed that playing Minecraft 1.16+ in my singleplayer world is sooo laggy at the beginning but after like 5-15 minutes the lag disappears and my cpu usage dropped significantly then I ran the tests again(in another world) and saw that before the transition, the gpu usage was like 20% but after the transition the gpu usage got up 80%. This also affects multiplayer
I know Minecraft is a CPU Intensive game but this never happened in 1.15+ cuz 1.15+ always prioritizes the gpu on boot up.

I also understand that my specs are really old(like my laptop is still standing since 2008) but it still runs pretty decent in the latest versions of the game.