Technical rant time...
tldr the technology behind the two airlines is totally different but also is the number of users.
Virgin Australia's wifi is a 2Ku band solution from Intelsat (In fact it was always a 2Ku solution during the early days partnership with gogo and Optus but now it's Intelsat operated) whereas Qantas relies upon a Ka band solution powered by the NBN network.
The current Intelsat antennas that VA have are a dual phased array where nothing moves inside the antenna (it's 2Ku meaning the dual Ku band antennas). Intelsat claim that their current 2ku antennas can achieve around 70mbps and they've got three main satellites that provide ku band internet coverage to Australia (They are no longer using Optus satellites). The 70mbps you've seen indicates there aren't many other people using the connection and the aircraft is getting close to the max speeds. This is kind of expected when such a small number of people onboard get it for free and everyone else has to pay $20-30 depending on the flight.
Qantas on the other hand currently have a Ka band antenna on their 737s and domestic A330s which connect only to the two NBN satellites. Ka band internet is higher capacity for serving more people faster speeds in beams which is a big reason behind the NBN opting for it on their satellites however it doesn't have the global coverage that Ku band does. Now the NBN Ka band can deliver the a similar bandwidth (or in most cases more) to the aircraft as VAs 2ku solution so the slower speed isn't the tech but actually the usage.
Just last week Qantas stated that an average 75% of passengers login to the wifi and Qantas claim this can be 100% as multiple devices are connected. Apparently a quarter of this is video streaming, let's say just 25 people are streaming video that consumes 3 mpbs (Netflix standard definition), that's 75 mbps just there and that's not even factoring in the remaining web browsing/audio streaming etc. Essentially the Qantas product is a victim of it's own success being free and usable, people are actually using it.
Now Qantas apparently had a cap of 12/1 per device but due to to high usage I haven't seen this for a while and no clue if it's still enforced. Additionally they have just started fitting the A330 international fleet with new Ka band antenna which allows them to take advantage of Viasat's growing global network of Ka band coverage (They are fitting the newer gen Ka band antennas before the satellites actually go live so it'll be ready from the first day the satellites are operational).
www.speedtest.net
