Thanks all for your suggestions, advice and also information Still learning, i never had a problem on Windows with the game so i'm glad that at least the issue came up, while i'm still at such an early stage and totally makes sense.
@Ashley : Thanks as well for your explanation, it makes total sense. Just for the record: i never blamed Construct or HTML5 or anything, i just thought it was more a question of Retina Resolutions than of GPU power so i thought maybe i was missing something.
The reason why i wanted to use the high quality setting on the fullscreen mode in the first place was, that i wanted to use Webfonts in my game. Now, at this resolution and going with the "low" quality setting, as you can imagine, you can't make out anything anymore of the text. But it works without any problems on my MacBook. It works rather well with the Settin on "high", the high-dpi display setting OFF and the scaling set to "letterbox integer"...
Which brings me to my question actually... What would you suggest to do? I see three possible options:
1. Go with Spritefonts, which i don't really want to, since the game is already being translated into languages using special characters like äöü or â and so on, plus again, with a Gameboy Resolution there's pretty much just so small as you can go with sprites.
2. Export all my assets double the res (512*288) and use Fonts there. They will still be blurry but at least less blurry than with half of that resolution.
3. I don't know, this is a long shot, but maybe i'm missing something? Is there a way to use the "low" quality setting on a project but still keep crisp Webfonts when scaling up?
I'm worried that when i upscale my assets, that i could run in performance problems again, since i'd like to use quite some effects (actually just one force own texture layer per layout, i think that second one was added by mistake )
About performance i got a question too:
When i, let's say, use my vignette png in half the resolution and then upscale it in the engine, does it use less performance than when i import a fullscreen vignette png in the first place?
Thanks for all your help!