It's almost entirely about screen resolutions. In the Turbo Pascal era the screen probably had 640x480 pixels or 307,200 pixels in total. The screen I'm looking at now is 5120x2880 = ~14.7M pixels.

If an app is maximized it has to render ~48x more pixels per frame, probably with anti-aliased text which older systems didn't use. Doing that on the CPU is not only visibly slow in many cases, but a waste of CPU time that could be better used for actual app logic. OpenGL happens to be a convenient and portable way to get the GPU to do work for you, at the cost of some memory, so GL it is.

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direwolf20 3 hours ago | parent [-]

It doesn't render them. Framebuffer native apps back then didn't have enough memory bandwidth to change every pixel every frame either. They just changed the ones that changed. You'd probably still let the compositor use the GPU to drag windows around. They had that back then too, it was called accelerated blitting.

	▲	mike_hearn 2 hours ago \| parent [-]
		GL based widget toolkits only render the needed pixels too. It's good for saving battery but isn't enough by itself as it only works until the app genuinely needs to repaint the entire window. Think a text editor or IDE switching to a different file.