Same here, I bought my Surface Book 2 from Taiwan last month.
And speedtest.net always result about 5Mbps/12Mbps while my iPhone 7, connected to same wifi and placed with Surface Book 2, reports 55Mbps/18Mbps.
This kinda wired that download speed decrease 10x than other wifi device and upload seems not effected.
Is this a bug in firmware or driver with this poor Marvell wifi chip?
Edit:
My wifi router is configured on channel 7 (2.4GHz, lower)
While connected to 5GHz (157 channel) with same router, the speed of download drops to half (about 3Mbps...)
Update:
After I configured wifi router to serve 2.4GHz wifi on channel 11 and enable coexistence 20/40 MHz (40 MHz only previously), speedtest reports 25Mbps/4Mbps (while iPhone gives 30/12).
Does channel and band really affected this chip?
I checked both channel 7 and 11 are non-exist wifi broadcasting on, this could not be wifi interference.