When you say all types of files, have you seen it primarily with images, or videos, or text files as well?
Is it in all folders, or just particular ones or types of folders? (Maybe network drives, or folders containing shortcuts, etc.)
If you turn off the thumbnail cache in Opus, does that make a difference? (Antivirus scanners usually cause problems with the caching side more than the generation side.)
Does that mean you can't turn them off on the new one to test that, or that turning it off didn't help and also caused other problems?
If the issue is with icon handler shell extensions, it's likely to be only one of them that's causing the problem, while the rest can be left enabled.
The icon overlay extensions are also involved here.
You could try making some Process Snapshots while thumbnail generation is happening and slow, which may let us see where the bottleneck is.