Sorry for the late response.
Do you experience this on every preview browsers ?
-Yes, every preview
Did you test it under export, NW.js included ?
-Yes, same thing for exports, even with NW.js
Have you got two graphic cards, (...)
-No, I only have one graphic card
Do you develop from a laptop computer ? (...)
Do you have a lot of iterations through objects (loops) that occur each tick in your C2 project event sheets ?
-It doesn't matter. FPS drops occurs even on an empty project with no code in event sheets
Did you look at the number of objects being created while debugging ? Maybe some objects are not destroyed.
-Again, it can even be an empty project with no objects in it
Are you on Windows 7 and not using the Aero (graphic interface) ?