Arrows shot from skeletons do not de spawn over time Normally, these disappear after 5 minutes of sitting like any dropped item, xp orb or similar entity. These count as entities in the world, so they can contribute to entity lag like that of item frames, minecarts, glitched items or other things.

I have a skeleton mob grinder and they shoot me on the way down to storage
I've tried to make a arrows de spawn in many ways but they aren't working
the skeleton's have shot about 700 arrows at the wall and as you can see I get about 2 fps when looking at the wall
pouring lava on the wall make the arrows disappear but they are still counted as entities