Have used Norton for years, doesn't slow anything down. If you have more than one computer you want it on, I think you can get a discounts on buying it in groups of three or five. Check out the Amazon pricing and download it with a key they send you after buying it.
If you do have a very old computer and it slows you down, I believe you can set it to do the daily scan sometime during the night, but you'll have to check that. I don't generally turn my computer off.
Bill
Nodwell RN110 out moose hunting. 4-53 Detroit, Clark 5 spd, 40" wide tracks, 10:00x20 tires, 16,000# capacity, 22,000# weight. You know the mud is getting deep when it's coming in the doors.