Ewido works fine with avast as far as I'm aware, as to if it will slow your system, that to a large extent depend on your system and if you were using the resident scanner. I don't use the resident scanner part of it (which after the trial period can't be used with the free version unless you buy the plus/pro version) only the on-demand scanner.
Adware is not truly a virus it usually redirects your browser home page or web pages to their sponsored sites or imports ads or creates pop-up ads, etc.