I bought a laptop last summer and a desktop just after Thanksgiving. First thing I did was remove Norton from both.
I had too many previous bad problems with versions of Norton conflicting with other programs.
Now, I use MS Essentials and have had no problems.
The improvement you experience may be in part due to a larger footprint of norton - in memory and in cpu time, and/or it may also be to “improvements” that Microsoft has made, that it has kept to it self, concerning the requiremnets of an api (application program interface - the set of exe and dlls needed) for ANY antivirus app to work better.
The MS “desktop” near-monopoly is decreasing due to the increasing power and verstility of tablets and “smartphones”.
MS will do anything they can to near-bundle any desktop apps they can into the MS-OS orbit or package; including making OS improvements specifically for specific kinds of apps and then not sharing those improvments with the app development industry - making it appear their app alone (on its own) is “better”. This is what MS has done throughout their history.