Thank you for your answer.
My screen refresh rate is 60 Hz, the website you've linked return 60 frame per second, and so does the scene inside the engine - Engine.get_frames_per_second() return 60.
However the ghosting test on testufo.com seems positive. I didn't know what ghosting was so I made some researchs. From what I understand, it would come from my monitor ? I find it surprising, if that's the case, since I've played a lot of pixel art games and never have that problem before. Plus it's not like the monitor is old (I use https://iiyama.com/gl_en/products/prolite-xu2292hs-b1/)
Another thing that troubles me is that at low UFO speed, I have no ghosting at all on testufo, while the blurry effect persist in godot even if I set the movement speed to a low value, plus the movement is strangely irregular and jaggy, which ghosting does not explain, right ?
Anyway, is there anything I can do about ghosting in godot ? With a small display window resized in full screen, the motion blur is really horrible...