80% Read Random Mixed Workload
The programs running in your task viewer mostly update log files. These almost always fall into small 4KB to 8KB random writes. Metadata is in the same category, and is almost always updated alongside sequential reads and writes. Incidentally, this is another reason why 100% loads are often more fantasy than reality.
With a random 4KB workload, the two sets of drives perform similarly up to a queue depth of 16. At that point, the 512GB SM951 plateaus as Intel's SSD 750s continue to scale.