I think a lot of the time when people say the network is slow. They really mean their backend is slow.

With a fast backend ~1-5ms response times (not even that fast). Streaming compression over something like SSE to keep your response sub 1kb packet (roughly an ethernet MTU).

With a push based model, pushing data to a user is half their RTT latency. They will only experience their full RTT on actions they trigger.

Now the network to you is distance to the server (not your rail/nextjs backend taking 400ms). Things like 4G and 3G are fine. The real problem is when you have such bad signal you effectively have no down or up.