Pretty significant and expected due to encryption. Using APFS (encrypted) and I get write/reads of 148/404 MB/sec. So slightly faster write speeds but about the same read speeds just by changing how it's formatted. I then reformatted it using APFS, I get write/read rates of about 285/497 MB/sec. The OWC Electra 6G, formatted in HFS+, gets write speeds of about 265 MB/sec and read speeds of about 503 MB/sec. The speed test should be roughly comparable to what they'd be if they were an internal drive thanks to the nature of USB-C. Ok, so I'm testing a pair of SSDs via a USB-C external enclosure on my 2019 iMac. The speeds should be close to what they'd be internally on the newer Mac. I have another one of these WD Blues in a USB-C enclosure, so let me test that for laughs on my MBA or iMac (also 2019) to see what results I get, along with the old OWC drive that I just pulled out. Even by WD's best case scenario, their drive is still half the speed of the one in this MBA. But this is much faster hardware, including the SSD itself. For comparison's sake, the Apple SSD in my 2019 MacBook Air has write speeds of around 1000 MB/sec. How the drive is formatted (APFS vs HFS, for example) may impact it also.

Reaching that speed depends on a lot of variables, and the hardware in the 2014 Mac mini may not support that. Bear in mind that WD hedges the specs by saying 'up to' 500 MB/sec.

I get similar results using Blackmagic's software, so rest assured you didn't get a lemon. Coincidentally enough, I just put that exact same SSD in that same model Mac mini.

Best ssd for mac mini 2014