How big are the PDFs that lag?
Not large; five to 25 pages, up to 500 KB. Mostly scientific journal articles; primarily text, a few tables or grahps. No photographs.
Odd. I know for a fact that RightZoom will have no effect on this, since it is only a UI mod for maximizing windows with the green button. Besides, you don't have it installed.
There's an app called "AppZapper". It completely removes an app and all associated files (preferences, plugins and .plist files)
Download it, Install it, and open it.
Drag the Reader app to the window and "Zap it".
Empty your trash and then download and reinstall the offline installer.
Then open a PDF the was lagging. Let me know if it persists.