How's an iGPU faster than a dGPU? In theory, it should be faster since it's integrated into the CPU but this splits the memory in half and therefore you can't utilize the VRAM fully. If you have a dedicated GPU, you can utilize the full VRAM.
More info please if you think "I'm not making any sense".
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So theoretically, sure if you have a dGPU on machine you get an extra 2GB of VRAM dedicated to the GPU, where as an iGPU has to share with the CPU RAM. But when you have 16GB of RAM, Ram limitations become no longer an issue. You can still have the CPU using 10GB and the GPU using 6GB. Or the CPU using 12GB and the GPU using 4GB. This is why dedicated VRAM does not give major performance gains.
Now in practice, we see from benchmarks that iGPU systems using only 45W of power perform on par with systems that have a 45W CPU and 50W GPU while using less than half the power. And saving money by not having to pay for 2 high end performance chips. And saving physical space.
This is why Apple is moving away from dGPUs.
Now don't get me wrong. I am an avid gamer and don't get me wrong I will still love to buy GTX Titans and stick them in my desktop computer. And I would even buy a 17" thick gaming laptop with a 145 Watt 980M that would have to be connected to a power source often. So of course there are some dGPUs that are going to still outperform iris 580, but that's because they are in another category, far higher price point, and far higher power consumption. If Intel wanted too...(I don't think they do) they could make an extreme end expensive 100 watt + mobile CPU with an even more powerful iGPU that could compete with things like 980M.
If the iGPU can accelerate my 4K video editing and compositing workflow better than a dGPU layout, I'm all for it.
I'm not against it...
It should be able to, and if not this generation than it will next generation.
Intel has been closing the performance gap between iGPUs and dGPUs dramatically over the last 5 years. And now they have
closed the gap. Iris 580 is faster than the dGPU in the 15" MBP. And Iris 580 is faster than any dGPU available today that fits Apple's thermal requirements for the MacBook Pro. Sure maybe this summer pascal and Polaris will come out with faster chips, but then those will have to compete with kaby lake iGPUs eventually. Now that intel has closed the gap, now going forward iGPUs will be come the faster than dGPUs.