I'm nearly ready to submit my game to the Mac App Store, but have run into a really major problem during beta testing. Some beta testers (about half it seems) are experiencing extremely slow fullscreen, while windowed mode works just fine -- even at high resolutions.

Initially, I tried reducing the resolution while fullscreen, figuring maybe a high resolution was the cause. But this and other attempts did not resolve the problem.

It is sporadic, and some users have found that by toggling repeatedly between fullscreen and windowed mode, they can (around the 2nd or 3rd time) get fullscreen running up-to-speed.

Just how slow is it going? About 18fps when fullscreen. It should be easily doing over 200fps, and does on my system, and does for them too, when in windowed mode, even with the window taking up most of the screen. It's not a resolution issue.

I thought it was something I was doing until i tried modifying one of the SDL demos to go fullscreen. I simply modified the testgl2.c demo, adding the following right before  "/* Main render loop */"

	{
		SDL_DisplayMode displayMode;
		
			// Set the resolution used when going fullscreen
		SDL_GetDesktopDisplayMode( SDL_GetWindowDisplay(state->windows[0]),  &displayMode );
	
		SDL_SetWindowDisplayMode( state->windows[0], &displayMode );
		SDL_SetWindowFullscreen( state->windows[0], true );
	}

And while yes, I'm switching to fullscreen later, instead of using a flag at startup, the flag at startup method results in the same result in my tests. 

DO NOTE: This does not happen on my own system! (iMac Dual-core 2.0 GHz Intel 20") It happens on more than half of my tester's systems, and quite consistently. Their systems are even newer/faster than mine, so it's strange.

The frame rates one of my testers gets with the above is:

Windowed:
2208.47 frames per second

Fullscreen:
29.65 frames per second

Hopefully it's obvious that such an abysmally slow framerate is NOT due to resolution. But as I mentioned, you can change the resolution simply by setting displayMode.h = 800, displayMode.v = 600 or something similar between the SDL_GetDesktopDisplayMode and SDL_SetWindowDisplayMode calls above, and it won't help. It'll still be very slow fullscreen on my tester's systems.

It's difficult for me to test "potential fixes" for this, as it runs great on my two Macs, so I'd have to send any potential fixes to my beta testers to see what they'd find.

This is a big bug that's holding up the release of my game, so I'm hoping it can be fixed soon! I'm happy to help with whatever I can -- i.e. try potential fixes and forward them to my testers, and get back to you asap, if that'd help.

I'll post some system profiles of two of the systems that run it slow fullscreen in follow-up messages.