Since Avast is resident, it scans all processes, files, and data in real-time when your system is running. Real-time protection is what keeps your system protected.
Only way to fix this issue for good would be to turn on Passive Mode in avast troubleshooting. This would make avast not resident and not run in real-time but only on demand. No resources are used when avast is not resident but set as passive mode.
Then you'd need another antivirus to run as resident to replace Avast.
More information about your operating system, how old your system is, desktop or laptop, etc., will help.