Slow read/write speed (40MBps) with USB-C external SSD on Macbook Pro I had 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.

I got a Samsung T5 External SSD and even with one I have the same problem. I have seen many reviews of T5 on youtube with Mac which all were satisfied. Am I missing something? Do I need some sort of Driver for it? I’m running a 2018 Macbook Pro with MacOS Mojave
I own multiple Samsung T3's and T5's. They are incredible.

The cable is important, and unfortunately, it's impossible to tell which cables are intended for which purposes.

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.

Be sure you're using the short cable with the velcro bits when connecting to your T3/T5. For some of my Samsung T3/T5's, the old-style USB-A connector cable is the best one for performance. 

Download the (I think still free) Blackmagic Speed Test from the Mac App Store and check the actual performance. Then change cables and you'll see what I mean.

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.

Before you down-vote me, please do what I've suggested. I literally have a dozen of these things, each 2 TB/$500.