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You both have valid points. Blackmagic's speed test will not be accurate until you saturate the cache on the NAS appliance.
The NAS have 16G of ram so you will need to copy over several large files around 16G each. I would use a different computer attached to the USB3 interface. Then run the test on the thunder bolt interface with black magic while the files are being copying from USB3.

This will give you an idea what transfer rate and bandwidth you can expect during disk high utilization.

Some NAS have a option to disable the cache.