just did a fresh install of windows and followed the steps you had outlined.
so i guess im in the wrong forum now to follow this up but ill let you know what ive found.
everything was running perfectly, got the point of installing the basics and iCue but once i installed Razer Cortex & Synapse then launched a game the lagging returned to my AIO LCD (browser media was still running fine tho). tried disabling some Razer Features (closed Synapse) and discovered it only happens when Cortex "GAME BOOSTER" boosts with a game running. (fixes itself once game booster turns off).
you can choose which services/process/apps or changes you want to activate when it boosts and upon taking a close look at the default settings i noticed an option called "Enable CPU Core" which focuses CPU processes on gaming specifically. so this was the culprit, simply unticking this and using boost fixed all lag issues for me. which makes total sense lol.
thanks again for your advice, though i was hesitent to reinstall windows it was the best option to narrow this down.