Just got a SB2 (13" i7), and I'm having the same experience after upgrading to version 1809.
It's frustrating, but what works for me is disabling and re-enabling the network adapter (Marvell AVASTAR Wireless-AC Network Controller), and then checking my speed at speedtest.net, I have to repeat this 1 or two times until I get high network speeds at 5Ghz.
The difference is dramatic, from 8Mbps to >120Mbps when fixed.
You can use the PowerShell command below to restart your WiFi adapter in the SB2:
Restart-NetAdapter -Name "Wi-Fi"
The PowerShell window must be in administrator mode, try this until you get decent speeds in speedtest.net.
Also, my WiFi system is an Ubiquiti Networks AmpliFi HD.