Just writing that I also have severe lag issues with the new OpenFL 3.5.0 and Lime update 2.8.0.
I am using the dev branch of HaxeFlixel. But not Openfl and Lime (is there a dev branch there?)
I also have really weird issues when toggling to fullscreen with alt-enter.
If I compile in windows or Neko with the -DNext flag there are no lag issues, but there are still fullscreen issues. I start windowed, but when toggling to fullscreen a black screen happens, if I toggle back, the window cannot go back to its windowed size and many weird artifacts, glitches and missing sprites appear on the screen.