Well the point is that by doing rendering on the client side you get most of the latency issues out of the way on first load and actually have a lot more control over its effects on the user even after that (e.g. you have the option of syncing with the server in the background).
1. Controlling how long page render takes at the server is under the developer control.
2. The size of network load is under developer control.
3. Client side optimization are under developer control.
OP is saying 1 is low; 2 is low(not much difference between html load and json load); and 3 is low as there isn't much going on at client side.
Other than that:
1. User network is slow, hence leading to large load times.
Go bake potatoes - can't help.
2. User is located at a distant location from the server.
If the user is part of a market which the company sees as profitable, the server can be mirrored.
Or else you can go bake potatoes.