This is correct. USB C cables vary greatly in specification. Some are USB 2 speed (480 Mb/s), some are 3.0 (5 Gb/s), and others are 3.1 (10 Gb/s). There's also a similarly wide spread in how much power they can carry. Apple's charging cables are rated for 480 Mb/s and 87 watts. To keep them thin while carrying higher current, they cut back on data speed.That speed would indicate for some reason running at usb 2 speed? check what system report says for usb on it.
What usb c cable are you using? I believe some usb c cables will only do usb 2, including possibly the apple charging cable?
Thanks for the feedback folks. I used 2 cables, the one that came with my mac and one that came in the box. I also tested with an old enclosure I had at home and the result was almost 10x faster. I suspect the enclosure I purchased is bad.