Hello @KPranger
AC1200 2x2 means – at most – two streams of 433Mbps each in the 5GHz band, for a maximum of 866Mbps. But, that assumes you're in the same room and right next to the access point and using a client that has support for at least two streams. It's also before taking into account the fairly substantial overhead in WiFi. 500Mbps is not unreasonable for something like a phone, tablet, or even most laptops. So, I am not surprised at that result.
Next, we need to cover how the secondary points talk to the primary. They use a hidden WiFi network running an 802.11s mesh to talk to the primary (and each other, if required). But, this network is using the same 5GHz radio as the units are using to talk to nearby 5GHz clients. So, any 5GHz traffic through a secondary is going through that secondary's 5GHz radio twice, and using the same 5GHz channel twice. So, the available throughput will be lower. That client might be talking to the nearby secondary at the maximum speed, but the secondary definitely isn't going to be talking to the primary at the 866Mbps rate, since it's a room or two away through floors and walls. I'm actually impressed you managed 300Mbps through a secondary at that distance, to be honest.
The 60Mbps number is a little surprising, but as you move further away from the secondary, the client will have to talk to that secondary at a slower speed (and as we discussed above, the secondary is talking to the primary at a slower speed already). Still, I'd open up the Google Home app and check the details for that client to make sure it's connected to the secondary you think it is. If it's an Android phone (from certain manufacturers), it might not be handing off between access points, but if it's an iPhone, it really should do a good job of that, though you need to give it 30 - 60 seconds to initiate a hand-off.
As for the third point, I would say you may be running into an attribute of the mesh network. The path selection algorithm prefers fewer hops, so if that third point can get through directly to the primary, it will do so rather than going through the one on the second floor. Since there's only one 5GHz channel, this is often better than adding extra hops. This is why the placement advice for optimal performance is to put the primary as close to the center of the home as possible and then place each secondary no more than one or two rooms away from the primary – close enough to get a strong 5GHz mesh connection. From there, the secondaries will provide both 2.4GHz and 5GHz coverage to more distant clients.
I know this all sounds complicated, and under the covers, it definitely is. But, with optimal placement, you should be able to get reliable coverage over a larger part of your home with a mesh system than you could with a single traditional access point. WiFi is a convenience technology, though – you're not going to be able to saturate a 1Gbps internet connection through an AC1200 system. But, the primary does have a 1Gbps LAN Ethernet port you can use to build out a wired network in your home, and that network will be able to saturate your internet connection. Running as much of your bandwidth-intensive traffic over this Ethernet network (e.g., streaming TV, downloads, gaming) will then preserve as much of your WiFi capacity as possible for those devices that can't be wired.
If you really want to optimize your WiFi network as well, it's possible to connect the secondary points (assuming these are Google WiFi) to that wired network. This would allow you to place the primary and secondaries anywhere you want, since they will be talking to each other through Ethernet instead of the 5GHz mesh interconnect. At that point, performance should be the same for clients near any of them. I've done this in my home, since my cable modem is in the basement. I have a wired secondary on the ground floor and another wired secondary on the second floor.
I guess the summary here is that if you really care about performance, you may need to experiment with placement in light of the information above – perhaps even going as far as running some Ethernet cables.
