Avast Web Shield slows down PC and internet.
I always choose custom setup (installation) and uncheck everything but the file shield.
I use Avast Free Antivirus in combination with the free version of Malwarebytes Anti-Exploit (real-time protection).
Malwarebytes Anti-Exploit shields browsers and software programs from attacks that exploit vulnerabilities in their code. A layer of protection that stops malware before it's delivered to your door.
I also use Malwarebytes Anti-Malware and ESET Online Scanner (installed on my PC) only for on-demand scanning.
The Webshield is probably the most important shield in Avast.
Since 85% of all infections come through the internet, it's the one shield that should never be turned off as long as your online.
It also uses very little overhead. I suggest you see what else is running that might be slowing you down.