1) What happens to all the arrows scattered throughout the game? Do they eventually disappear?

1) Ashpiles... Do they, too?

I have fear that these 2 things would become a reason for the game to lag at certain points.

Oh, and if someone would be so kind to post a link to a place (if it actually exists) that lists or generally talks about the things that could make the game lag so that I can avoid them, would be great.

Thanks in advance.

Nope, as far as I know, they're there till the end of days.
The ashpiles were bugged in Fallout 3 too, seems they never fixed that issue.

theres fixes for em.....course getting the fix depends on whether or not you're on a console. you can easily get a clutter clean up off steam or nexus.