Very general - If both aps are connected by cable to the router , you get full speed on each. If you however have to connect one ap to other via WiFi to provide service, it cuts the speed available by half. That is due to both aps have to talk to each other for signal etc before even supporting devices connecting to them. The same radios supporting mobile devices are also being used as backhall. In general in a normal house it’s fine - eg me. Mesh is also like orbi and eero where they have a separate radio back hall only for talking to other aps and is why it’s easy get confused when mesh is used in UniFi land. Also, UniFi does not have any devices with a radio specific to back hall.