I'm having great perfs on my laptop (chrome on macbook pro 16") but poor perfs (freezing every 30s) on my Chromecast ultra.
Same wifi (5Ghz), same distance from the router.
When doing a M-Lab test, I get 35 Mbps.
I wish I could but my tv is far from my router (~6m).
I'll try to find a really long Ethernet cable...
But why is it completely fine on my laptop and Pixel 3?
Same wifi, same location... Is it because my TV is bigger and 4k?
I've had trouble just streaming video over WiFi to a Chromecast attached to the back of a TV before. It wouldn't take much for the TV's electronics to interfere with the Chromecast dongle's WiFi signal. I actually found that using a short (3-6") HDMI extension cable could really help with that.
Switching to a wired connection completely eliminates the possibility of such interference; if that's not possible then you might want to see if an extension might help at least a little bit.