The amount of data that can move over wi-fi is much less than the "theoretical" speed because of "overhead" factors such as the devices co-ordinating their signals back and forth.

As more devices share the signal, resulting in more contention to be negotiated between the devices, the amount of data that can be transferred drops further.

Other wi-fi devices in the neighbourhood may also add to the contention, further detracting from the ability to get actual data sent over the wireless connection.

The full speed of the NBN requires a wired connection within the premises, which can run an order of magnitude faster than wireless.

Plus, each wired connection is independent, with no airwave contention impeding the ability to move data over it.

To improve wi-fi performance, consider at least running wired connections to stationary devices such as TVs.

With a reliance on just wi-fi within a home or business, the wi-fi itself is probably the bottleneck. There will likely be little difference in performance between ADSL2+ and NBN connections.