Ihad a Sandisk Extreme Pro 500Gb PCIe NVME M.2 SSD. I got an USB-C 3.1 Gen 2 Enclosure for it. I have tried it on 3 different 2017 and 2018 macbook pros and an iMac. The Max read/write speed that I can get is 40MBps. I thought the problem might be with the enclosure so I tried 4 different enclosures and still same result. I installed the SSD through PCIe to a desktop and read/write speed were astonishingly high.
Your 2018 MacBook Pro with Mojave (I have a 2017) does NOT need any kind of driver to use the drive. There's an encryption app on the drive that's worth installing so that you can secure the contents, but that's orthogonal to speed.
As a rule, if the cable is more than 6 inches long, it's NOT for best performance. If the cable has the Thunderbolt symbol on it, it MAY be for performance, regardless of whether it has a USB-A or USB-C connector on the business end (toward your Mac). Also, DO NOT connect your T3/T5 through any kind of cable extender or dock. There's a distance limitation that you will run into if there's any kind of middle man device.
Had the same problem just now with a Samsung 970 EVO Plus. Was using a 2 ft USB-C cable, and only getting 40 MB/s. Swapped out with a shorter cable and getting over 1000 MB/s. I should be seeing higher numbers though, so I'm going to purchase the highest quality cable I can find. Definitely the cable in this instance.
Problem was quality of the cables. the cables were using the USB-C connector, but as you may know not all USB-C cables are equal. most of them are only intended for charging at low speeds and are not suitable for hight speed data transfer because they don't fully support USB-C specifications. They only use the connector. ( most probably with less wires inside the cable than required. not all pins are connected )
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