The only thing Avast does at login is update the data base like any other antivirus does. Unless you have a scheduled scan set up. It could also be a software conflict of something that’s running in the background. But only you know what is running on your pc. You can add exclusions into Avast. Exactly how many programs do you have on startup? How many items in your system tray? I only have 6. Avast being one of them and my Google Drive.