Browsers can slow down a computer's functionality. I use Firefox as my default Browser, but also have installed Internet Explorer, Sea Monkey and Safari. The fastest is Safari, with Firefox next, followed by Sea Monkey with Internet Explorer being the slowest. There was a time I was using Google Chrome on occasions, but have removed it as being a bug breeding browser.
Make sure you have completely un-installed AVG but please,please,please, Do Not become involved with CCleaner which could, if Not used extremely carefully, do irreparable damage to your computers Registry and even render your computer beyond further use. Far better to give your computer a Scan with the Avast which you have installed and then a quick Scan with the free version of Malwarebytes. :-
https://www.malwarebytes.org/free/
Hans.