@ robin183
It certainly isn't normal.
On my system Avast only has one process in view in my task manager screen and is very low in all regards. Firefox is normally my highest CPU& RAM user.
Do you happen to have any major programs running in the background ?
Or any other security based programs ?
You say you have tried many options:
Did you try an Avast Repair followed by a system Restart ?
I know this is a step down from an uninstall and reinstall, but it is actually looking for issues.
Having done an uninstall and reinstall, was this a clean reinstall utilising the Avast Clear program.
Clean Reinstall - 1. - Download the avast! Uninstall Utility, avastclear.exe
https://www.avast.com/uninstall-utility and save it to your HDD. Make a note of the steps required or print the page.
2. - Download the latest Avast off-line installation file
https://files.avast.com/iavs9x/avast_free_antivirus_setup_offline.exe and save to your HDD.
Follow the instructions in the link in 1.
I believe there has been at least one minor program update since then so another restart should check for and install any minor update.