the QNAP TS-831X is a terrible unit (I know, I own the TS-531x). For the same money, you should have purchased the QNAP TVS-873e, which is fast and wonderful.
The TS-831X has a native dual port SFP+ 10G card in it. This will never go faster than 200 MB/sec. So when you say "I am connecting this to my MacBook Pro via the Sonnet TB3 adaptor" - the only way you can do this is (since the Sonnet adaptor for the Mac Book Pro is the Sonnet Solo 10G, which is a Cat6 RJ45 ethernet connector, not a SFP+ connector like the TS-831X comes with for free) - I assume that you purchased a 10G card for your TS-831X.
Is that correct ? There are few cards from QNAP that are compatible, and the one that is compatible - the QNAP
LAN-10G1TA using the Tehuti 10G chip, and it's slow as well. So you have a slow card in a slow box. And that's IF I am making the correct assumption about your system. You have not stated what 10G card is inside your QNAP.
Either way, if your QNAP is configured correctly, you should be getting much faster than 20MB/sec READ. There are native 1G ports on the TS-831X. Have you tried those ? You should get 100 MB/sec just from those. Of course, you must have SMB signing disabled on your Mac for this to work.
OK, let me know. I am here to answer your questions.