Apple compares the M1 Max as having similar performance to a nVidia's 3080 Laptop GPU, which scores around 16,500 on passmark. For comparison, the AMD 6900 XT desktop CPU scores 27,000, while the nVidia 3080 desktop GPU scores 24,500.
So the M1 Max is not as fast as a high end desktop GPU. Still, it is incredible that you are getting a GPU that performs slightly less than a last generation 2080 desktop GPU at just 50-60 watts.
Yes, apple's marketing materials claim 400 GB/s, while the 6900 XT is 512 GB/s. This is very easily Googled. While memory bandwidth isn't everything, it is the major bottleneck in most graphics pipelines. An x86 cpu with 3200 MHz memory has about 40 GB/s of bandwidth, which more or less makes high end integrated graphics impossible.
Ah, I misunderstood your comment. When you said it was competitive with the 6900 XT I thought you were talking about GPU performance in general, not just in terms of memory bandwidth.
According to the numbers Apple is touting, the M1 Max is competitive with modern GPUs in general, being on par with—roughly—a 3070 (laptop version) or a 2080 (desktop version). They've still got a ways to go but this is shockingly close, particularly given their power envelope.
So the M1 Max is not as fast as a high end desktop GPU. Still, it is incredible that you are getting a GPU that performs slightly less than a last generation 2080 desktop GPU at just 50-60 watts.