I have a very high end Internet product from Comcast. I just ran a speed test. The result is 278.5 Mbps download. My router is about 10 feet from my Roku. I have restarted the device several times in the last couple of days. I just bought this Streaming Stick+ about one week ago.
I am using the power supply that was in the box. The system updated as soon as I plugged it in and that was last weekend. It is using my WiFi. I don't know how to answer your last question.
The SS+ (3810) can use either 2.4Ghz or 5Ghz - depending on which band it is using/connecting with, it may impact the performance of the device.
If your modem/router/gateway supports both, they may either have the same SSID, or different SSIDs (I recommend separating the SSIDs to control which devices go on which band). If it has different SSIDs, one is usually named "xxxxxx-2G" and "xxxxxx-5G" by default.
Some ISPs also disable the 2.4Ghz SSID of newer modem/router/gateway models - I highly recommend re-enabling this (you may need to contact them to do that).
You need to check Settings/Network/About to see if your SS+ is connected to a 2.4Ghz SSID or a 5Ghz SSID, as this may directly impact the performance of both streaming and navigating the device.
Otherwise, if switching WiFi bands doesnt address the performance issue, you may have a bad device and want to return/exchange it.
On my older Roku external devices including the ultra it is quite slow. I would have thought it would have been faster in changing channels on YouTube TV with a new 2021 Roku but I guess not from what I read here. I did try YouTube TV like others had on an old fire stick and it changed channels twice as fast. So I believe the issue lies with the Roku device.