Disabling all modules in Avast isn't a great way to test as it doesn't pinpoint a particular shield/module.
You could test by first disabling the Avast Firewall and enabling the Windows Defender Firewall (which is disabled by default when you have the Avast Firewall component installed) and see that makes a difference.
However, I specify that the slowdown is not much, it is barely noticeable when surfing the Internet and only by doing the ADSL speed test comparing the values it oscillates with a difference of about 30 Mbps. It is certainly due to the WEB module in fact by disabling and enabling only that module I notice the difference.
Probably like all AVs the WEB module for the checks it performs slows down the connection slightly and this is evident especially with old CPUs like mine.
Here are the characteristics of my CPU:
Intel(R) Core(TM)2 Duo - T7500 - 2.20GHz