DIAGNOSIS This is usually a suboptimal mesh wireless backhaul or a device incorrectly connected to a distant point. Speeds degrade over each wireless hop.
FIX Test speed wired and wireless at the primary router. Connect a laptop via Ethernet to the router's LAN port. Run a speed test. If wired is slow, the issue is your modem or ISP. If wired is fast, test Wi-Fi speed standing next to the primary router. If that is slow, in the Google Home app disable "IPv6" under Settings > Advanced networking. If speed is only slow at a point, reposition that point closer to the router.
IF THAT DIDN'T WORK The 6GHz backhaul may be disabled due to radar interference (DFS). The system silently falls back to 5GHz for backhaul, cutting bandwidth. You cannot manually control this. The only fix is to use wired backhaul: connect an Ethernet cable from the primary router's LAN port to the secondary point's WAN port.
HEADS UP The "Network Check" in the Google Home app tests connection health, not maximum throughput. Always use a third-party speed test app like Ookla for accurate speed measurements.