avast scans http web pages/traffic before they are saved/displayed in your browser. However it should have no adverse effect unless you are on very fast broadband (100Mbps). I see no impact and my connections (on testing) is in the region of 37Mbps.
What other security software do you have installed, e.g. firewall, anti-spyware (previous AV), etc. ?
totally unrelated to avast, it doesn’t change file attributes. The only files it would protect are its own program files.
Seems to me if some (all?) files on your computer have been changed to read-only, this could affect writing data to the hard disk? Possible reason for the slowdown? Did you do a system restore recently?
See these two websites and read through both; ownership of files may be an issue or UAC setting level?