I have seen the update page you linked. There are also updates for version 4.0, which total 3.8 Mb.
I don't know if the current VPS will work with your old program. You may want to wait and see if anyone else here knows.
I do know that over time, updating my Avast versions have not caused any performance hit to my computer. The current version is no more resource-heavy than the old one you are using, I would think.
Slow and sluggish computers can be caused by lots of things, apart from age. More RAM is often an answer. Preventing programs that don't' need to from starting with Windows is important, especially with a low performing computer. Carrying out routine maintenance is important, such as cleaning the disk of junk files, and defragging it. There are various tricks.
What is your OS, and your RAM/CPU specs?