So, I had a similar issue with a my Surface Pro 5. In my case, it would only happen when running on battery; with the power cord connected speeds would be over 100MBPs.
It was a battery configuration:
To start, open Control Panel then open Power Options; now you have to click on Change plan settingsright in front of "Balanced (recommended)" - below " Plans shown on the battery meter - after that click on Change advanced power settings now all you need is to find (Wireless Adapter Settings => Power Saving Mode => On battery) and change On battery to Maximum Performance. That's all.
To finish hit the "OK" button and restart (I am copying it from another post, not sure how to give credit).
In my case I could not find the option for Wireless adapter, so I had to "unlock it" I follow instructions from here:
https://surfacetip.com/how-to-unlock-power-plans-on-surface-devices/
It all worked after restarting the device.