Hi Peter,
The Samsung X5 has a severe thermal design issue, it does not dissipate heat as it should, so it throttles down quite quickly.
In my case, in a typical summer day with 30 deg C, the measured write speed was around 150 MB/s, but on heavy use it went as low as 50 MB/s (which is comparable to a poor micro SD card!).
I opened the case (watch the screw behind the serial number and the two screws behind the plastic part) and I installed a M.2 heatsink.
I suggest you run Blackmagic continuously for several minutes, and if you see a significant performance drop, you probably have a similar issue.
Edit: the heatsink somehow improved things, but under heavy use, the speed still drops to about 50 MB/s... Probably it requires a water cooled heatsink :-(