Avast is not the cause of this.

Can you give me your applied internet speed?
Download and upload?

I have problems with netlimiter pro. I dont trust that software.

You can test your connection using speedtest.net

Disable avast and test speedtest clicking the server location of your applied internet connection.

If the result of speedtest.net is not really the speed result of the applied connections of you paid then their is a problem with your internet subscriber.