First, you should always add the Addon Manager folder to the antivirus Exclusions.
And, it's not possible you won't see any improvement after adding the scenery folders to the A/V Exclusions, the only possible reason for that is you have lots of sceneries in a folder you still haven't excluded. Check the Documents\Prepar3d Addons folder, some developers put the actual files there, instead of just the add-on.xml.
10-13 minutes is completely undehard of, unless your hard drive is extremely slow and/or you have an insane number of folders. But again, the difference between having the proper file exclusions (INCLUDING the Addon Manager), should be about 10x faster so, not possible not to notice.