My question: the web shield can slow down the speed of my net?
What version of Avast are you using? v.5.0.594 is the latest build.
What other security software do you have on your machine including FW?
Have you checked to make sure you are malware-free?
Speed can vary depending on what you are doing at the time since Avast will scan to make sure you are clean. In addition, make sure your FW is not blocking anything with Avast. But I need the above questions answered first. Likewise, please give me information about your machine OS and RAM, etc.