Having the same issue on two similar Roku Premier streaming devices. Both devices are exhibiting the same problem. Up until a couple of days ago I was seeing excellent "signal quality" with typical "Internet download speed" of around 36Mbps-42Mbps. It has always varied a little from test to test.
Now over the course of the past couple of days it's dropped to 5-7Mbps at best. (I noticed Netflix and Disney menus were sluggish re populating) Nothing has changed re any new equipment or location of devices throughout the home. I've checked my router and I'm seeing 72Mbps via the ADSL connection. Every other device attached to the Wifi is seeing speeds varying from 30-48Mbps depending on distance from the router.
I've restarted the Roku boxes and Router several times with no improvement. Even if I move the Roku box within 6" of the router and the signal is absolutely maxxed out, download speed still sits between 5-7Mbps.
I've downloaded a little Wifi signal app to my phone and cant see any overcrowding or signal being swamped by other Wifi networks.
Forgot to add. My ISP is Plusnet. I'm in the UK