@Robert1394,
Old computers are usually slow by nature.
Old operating systems. Slow hard drives, lack of ram, etc
The less capable the computer, the more drag you'll notice
when a program like Avast which must have access and be able to scan
everything running on your system.
You can't turn of the program you charged with protecting your system.
If that were possible, how could Avast or any other reputable AV keep you safe.
One of the best and cheapest investments you can make to an old system besides replacing it,
is to update or increase the Ram and get an SSD hard drive. The SSD drive is probably the one
improvement that will get the most bang for the buck.