After discovering an issue with Google Home and Chromecast products was killing Wifi connections last month, Google immediately began working on a fix. A version of Google Play Services was rolled out to those on the Beta channel quickly but now it’s arriving for the rest of the world in the Google Play Services stable release.
Google Play Services stable release, v11.9.75 is now rolling out to devices and you should find an improvement to your Wifi network, if you hadn’t already unplugged all Google Cast enabled devices already.
The slow wifi issue was caused with Cast enabled devices flooding the network with keep alive MDNS multicast discovery packets, and this update should resolve that. The Play Services stable release is in broad rollout from today, but if you’re having issues getting it, you can find it over on APK Mirror for side-loading to your devices.