Hello @kclarke
Performance is always challenging to predict. From an Ethernet-only perspective, the primary Google WiFi unit (the one that's connected via Ethernet to your internet service) should be able to provide well above 900Mbps over Ethernet to an interior network through it's LAN Ethernet port. That's all assuming good quality cables and switches that reliably negotiate 1Gbps Ethernet speeds.
For WiFi, things are more complex. The WiFi radios in the Google WiFi units are AC1200 2x2, which means two 150Mbps streams at 2.4GHz (300Mbps total) and two 433Mbps streams at 5GHz (866Mbps total). The access point can talk to 2.4GHz and 5GHz clients at the same time, but clients are only on one of those. So, in theory, a 5GHz client that also supports at least 2 streams and is in the same room as the access point may be able to negotiate an 866Mbps link speed. Keep in mind WiFi has substantial overhead, so actual throughput is likely to be 80% of that give or take. From another room through walls or other obstructions, the negotiated speed will drop off significantly, especially for 5GHz. This is particularly important when thinking about the performance of clients that are connected to secondary Google WiFi units instead of the primary, since those secondaries are talking to the primary using the same 5GHz radio. This also has implications for end-to-end performance of 5GHz clients, since their traffic will have to get sent over that 5GHz channel multiple times at different speeds (depending on the best available link quality between the client and the secondary and between the secondary and the primary).
If you can connect the secondaries back to the primary's LAN port via Ethernet, you can skip one of those 5GHz hops, and provide clients the same performance as they would get when connected to the primary.
All of this adds up to thinking about WiFi as a convenience technology. If you want to go fast, you need to rely on Ethernet as much as possible. The WiFi mesh is a convenient way to expand your reliable coverage are, but you will be trading off peak performance in some areas in exchange.
