I don't know whether any of you know, but UGreen have just bought out an Ethernet adaptor for the original and new Chromecasts, and what a difference it makes! Despite not being too far from my WiFi router, I was really having stuttering problems with my original Chromecast in my garden office; it really shouldn't have 'cos the signal was good, but I think I was getting interference from the laser cutter and 3D printer running at the same time. Electrical noise really doesn't help with streaming via WiFi.
Anyway, I've been looking for a way to improve it for a while, until I came across this new cable. You plug in the microUSB to the Chromecast as usual, and the other end to the power, and the third 'end' (!) has an ethernet port! Worked first time! I was expecting to have to reset the device so that it would use ethernet in preference, but nope, it defaulted to the Ethernet connection. Now streaming is flawless!
Thoroughly recommend this if you've got the same problem.