I am noticing that the Ookla speedtest is not liking the 380.4164 firmware. The test takes longer than usual to start and I'm getting speeds in the 600 down 300 up range vs 945/945 normal. Router config is factory defaults with CTF and FA enabled. Other speedtest sites are working just fine. As a side note I'm seeing the same behavior on the Ookla site with Merlin 380.64 on this router. I didn't check any of the other sites while the 380.64 was loaded. The issue is not present on my AC3100 on 380.64.
Are you using the flash based test or the beta HTML5 test?
Flash based tests are known to be unreliable and is why all test sites are moving to HTML5.
My results with 4164 are right where they should be with the HTML5 test.