You are using Godot 3.1.1 Standard Version?
Which graphics card are you using and which driver version?
Perhaps there exists a driver for your card/chipset with a better OpenGL support.
How is the CPU-load?
Does this also happen when you start Godot with the GLES2 Renderer?
<godot-executablename> --video-driver GLES2
You'll see all parameters when your start the godot executable from the command-line with the "--help" parameter.