What you need is good debugging. What are EXACTLY the conditions to reproduce the problem? Does the lag also occur in other resolutions? What about in windowed mode?
A serious lag often comes from “On Tick” since that runs EVERY frame, so in most cases you have something consuming too much CPU every tick.
If it only occurs on very special conditions like “I set res to 1920x1080 in the game menu” then check that game menu, the problem will likely be there.
Also try to remember since when the problem occurs and what you changed since then.