Hello @mattnest99
This can be complicated to evaluate. When properly deployed (primary node as close to the center of the home as possible, secondary / point nodes one or two rooms away from there) the system will provide a larger usable coverage area than a single node would. So, depending on how far away the clients are, some of them may have higher performance than they would with a single primary. Clients that are very close to the primary should have the same performance they would if that was the only node, though, since they should be talking directly to it instead of going through another secondary.
However, there may be some regions where a client ends up connecting to a secondary / point unit instead of the primary, and the impact of sending that data over the channel twice (through the secondary / point unit) could under some conditions result in lower performance for that client.
Keep in mind there is only one 5GHz channel, so you want to make sure all devices can use that channel at the highest data rate possible, and that means being close (one or two rooms away at most).
Running eight cameras continuously streaming over WiFi is going to be a challenge for any system. The biggest improvement you could make to a system like yours would probably be to buy an inexpensive (unmanaged) Ethernet switch, connect it to the LAN Ethernet port on the primary Nest WiFi Router unit and then run Ethernet cables from there to each of the two secondaries. That will free up 5GHz channel capacity to carry your camera traffic since it will only have to go over the 5GHz channel once instead of twice. This also means you don't have to ensure the primary is located centrally, and you can even move your secondaries further away from the primary since they are talking to it via Ethernet instead of 5GHz WiFi. That may mean they are closer to your cameras, improving the data rate of those links. Here's a help article on this: https://support.google.com/googlenest/answer/7215624?hl=en
If these cameras are continuously streaming, you may be able to reduce the quality of those streams a bit as well. I have a Nest Hello camera that continuously streams, and I've reduced the quality of that stream a bit to deal with my Comcast monthly data cap.
I hope some of this helps!
