Ill see your “wierd” and raise you a “weirder”.
So playing around with the file trying to find the issue, I created a new level and dragged in the blueprint. Instant lag, After playing around some more, the Lag only occurs when the blueprint is actually selected. And it doesnt matter what is connectedf to the execution pins.
I literally disconnected everything from the begin play, spawn terrain and the construction script. So nothing was connected to any execution pin.