Hi Philip,
It is a good design practice on Wifi to separate services on different SSIDs, so you can tune each SSID with different parameters/features depending on the type of traffic (Video, Voice, etc) or even type of users (corporate, guests, etc.) In this case, just creating a separate SSID for the Chromecast you have seen an improvement since the type of traffic on this SSID is similar (multicast) and is more efficient for the AP to manage this as if there were different traffic scenarios (one user browsing, another user downloading a file, etc.).
Best Regards.
David.