Well just a general comment; Older computers are inherently 'slower'.
I have a dell laptop I purchased in 2001 with XP Pro. This laptop has used Avast since V.4 and now uses the latest stable version.
I can say without a doubt the RESOURCES needed to run avast has increased dramatically and while extremely noticeable on the xp SP4 over the years is simply not noticed on my HP Compaq.
Older is Slower.
My old Dell also only has 2 gig of Ram, my newer computers have either 4 or 8 gig. The HD's are also faster on the newer computers.
My old Dell has been upgraded to Windows 10 and is usable but certainly doesn't compare to the newest addition.
I'm personally not as fast at 75 as I was at 57 or at 21.
I'm also storing some extra baggage as is your old computer.