Why does the program slows down Internet connection? When the shields are active internet speed around 18Mbit / s. If you turn off the shields internet speed around 90Mbit / s.
OS XP Profesional SP3 CZ, Avast Free
Please try go to:
Settings->Active protection->Customize Web shield-> Uncheck “Enable HTTPS scanning”
and try the speed test again.
Martin
Some result …
…
That’s the main reason I asked for a refund on my 10 PC license / 3 years 
I agree that it slows down the internet speed. Avast needs to work on that. Especially when it still is not preventing infections.
Change the webshield settings one at a time and see if a specific one is causing the slowdown.
I do not understand what should I change?
https scanning has changed in avast 2016 and works a lot better on XP systems.
Update avast and see if the problem is solved.