Diagnosis
There are a lot of bottlenecks that can cause this. The most common for the Nest Wifi Pro is that you're testing with a device that doesn't support Wi-Fi 6E, so it's connecting on an older, slower band, or there's interference on the 6GHz channel.
How to Fix It
Run a process of elimination. 1) Test your internet speed by connecting a laptop directly to your modem with an Ethernet cable. This tells you the actual speed from your ISP. 2) If that's good, connect the laptop via Ethernet to the Nest Wifi Pro router. If that speed is slow, check your router's WAN cable and its QoS/priority device settings in the app. 3) For wireless, make sure the device you're testing with (like a new phone) actually has Wi-Fi 6E. Stand close to the router and run the test. If it's still slow, check for channel congestion in the Home app under WiFi > Settings.
If That Didn't Work
A firmware update might have gone sideways. Try a power cycle: unplug the router and all points for 2 minutes, then plug the modem back in, wait, then the router, then the points.
Heads Up
The Nest Wifi Pro's big speed claim requires a compatible Wi-Fi 6E client device, a broadband plan that's actually that fast, and ideally using the uncrowded 6GHz band. Most everyday speed drops are because one of those three things isn't in play.