Many users confuse "Play Services XML download" with a separate issue: the running under Play Services. If you check your network stats and see huge downloads (100MB+), that is likely not an XML file—it is an app update or asset delivery.
Play Services will try to download XML files, fail silently, and skip the operation. Push notifications and location may degrade because the same endpoints are used for multiple services.
When users see "XML download" in their network logs or firewall, they are usually observing . Google uses XML (Extensible Markup Language) files to update the behavior of Play Services and other apps without requiring a full system update.