Half decent laptop, doing very little.
Fibre broadband
Amazon prime video (so, a legitimate source)
Laptop a couple of metres away from router
Casting using a V2 chromecast
Only the laptop and phone connected to broadband

..but yet it always casts in pixelated, juddery rubbish.

WHY?!

Is it possible the wifi signal is suffering from interference? Try changing the channel on the router.

You’re a genius.

a) Googled what the hell a wireless channel was
b) Found out how to change it
c) Kept changing

..and result! I’ve been putting up with this crappy streaming for bloody ages and now it’s pretty much perfect; thank you very much 🙂

Laptop a couple of metres away from router

And just for future reference, unless you’re casting a screen (which you aren’t) the distance between the laptop and the router is pretty immaterial.
Once the Chromecast starts streaming it takes over all connection duties – your laptop/pc/phone then simply becomes a remote controller.

Hah you are welcome. I’m paid to know (mostly guess) stuff like that lol