When you run the Avast scan, does task manager show any other programs using CPU and Disk time? Maybe uninstalling Avast enabled Defender or something, and now they are fighting.

Avast has options to configure the scans (top right gear icon above the scan). For the full scan, do you have enabled the ‘scan all files’ option? And then below that: content or name extention.

Also: what scan areas are included? Maybe you customized that previously and they are now reset to default.