I don't know if the iFly data servers have the bandwidth to pour bits downstream as fast as the Microsofts and Netflixes of the world, but unless your device is very old I'd expect you to see better than 600 KB/sec.
I have a (supposedly) 500Mbps plan from Comcast. When I run speed tests using fast.com (a Netflix site), I consistently see 200 Mbps at my Win10 laptops, 180 Mbps at my Samsung S9+ phone, 120 Mbps at my Samsung Tab S2 tablet, and 120 Mbps at my Asus Zenpad 10 S3 tablet (all of which are connected via wifi). Divide by 10 for a conservative MBps rating (or divide by 8 if you prefer).
But I don't see anywhere near those speeds during iFly updates. I am guessing that's partly due to bandwidth bottlenecks at the iFly servers, and also overhead on the device side because there is some processing in parallel with the downloads (uncompressing files, updating screen, brief pauses to stop/restart data flow from one download file to the next, etc.)
And if it's been a while since I turned on my Android tablets (which is often the case), then they may automatically launch into a bunch of app updates in the background that can really slow things down, due to competition for both CPU cycles and wifi bandwidth.
I just ran a test, and saw these results (numbers came from observing the instantaneous speed reading on the update page while the download was in progress):
Samsung Tab S2: Max rate I saw was about 1200 KB/sec. During steady-state transfers of big files, that number stayed between 1100-1200 KB/sec, but dropped to 500-800 KB when it was chugging through a bunch of small files. Spent most of the time showing 1100-1200 during a 400 MB download.
Asus tablet: Similar to Tab S2 above, but a little faster. Big files would get up to 1700-2000 KB/sec. Small files were still in the 500-800 KB/sec range. Spent most of the time showing 1500-1700 during a 400 MB download.
Samsung S9+: Much faster than the tablets; speed rarely drops below 2000 KB/sec, with peaks around 4000-4200 KB/sec on big files. Spent most of the time showing 3000-4000 KB/sec during a 400 MB download.
Win10 laptop: Fastest: Speed rarely drops below 3000 KB/sec (though occasionally showed ~2500 when a bunch of small files in a row came through), with peaks above 8000 on some big files. Spent most of the time showing between 3000-5000 KB/sec during a 400 MB download.
FWIW, I generally see about the same performance on these devices regardless of whether I'm updating 1, 2, 3, or 4 devices at a time (though the tests above were run one at a time). This also points to the bottleneck not being at my internet provider or within my wifi network.