Three Macs slowed down just after I installed Norton One.
An older Macbook (2009), a Macbook Air and an iMac. All were on latest OS X software.
After installing Norton One all three ran Safari noticeably slower. With one, I didn't even tell the regular user but received complaints.
The most powerful system was a 32Gb memory quad core i7-3.7.
I have now removed the whole Norton One suite from these Mac environments. They have all speeded up again.
Draw your own conclusions.
Here's a link to the uninstall page
Update: I called Norton.
They told me they had not heard of this slow running before.
They suggested maybe I should try removing the browser 'add-in' for Norton?
As a quick experiment, I reinstalled Norton on the fastest machine. I ensured the browser extension was removed. The browser ran faster, but of course it didn't then have the Norton capability.