I don't know if this is a bug or what it is but when I fill in big areas with /fill it seems like it takes more computer power to fill it if it is high up in the world.

I tried these 2 commands:
/fill -64 140 63 63 140 -64 minecraft:stone
/fill -64 255 63 63 255 -64 minecraft:stone

The first one was executed in 10 seconds, but the second one took like 10 minutes to execute. I don't know why but can't really fill in the blocks that high up.