DIAGNOSIS The Wi-Fi radio in the Chromecast is weaker than other devices. It may be connecting to a congested band (2.4GHz) or a distant router.
FIX Force the Chromecast onto your 5GHz Wi-Fi network. Go to Settings > Network & Internet. Select your Wi-Fi network, then 'Forget'. Re-select your network from the list. If you have separate names for 2.4GHz and 5GHz, choose the 5GHz one.
IF THAT DIDN'T WORK Use the USB-C port for power and data. Buy the official Google Ethernet adapter. Plug it into the Chromecast's USB-C port, then connect an Ethernet cable from your router. This gives a direct, stable connection.
HEADS UP The Chromecast generates heat. If it's tucked behind the TV, thermal throttling can slow performance. Ensure it has some airflow.