Obviously you can interpret the parents statement in more than one way. My interpretation was as a rather pragmatic statement that the probability that Metal is a high performance design (and implementation) is lowered by the fact that it to date only has targeted lower performance hardware, and thus there could be several undiscovered bottlenecks present. It is generally the case with any code that high performance on less powerful hardware not always imply high performance on more powerful hardware, especially when there are high levels of parallelism and multiple optimised fast paths to choose from.