It will most likely be the load on the servers.
LOTS of people try to download straight after the keynote finishes.
Paul Watts
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This definitely looks like an interesting development (pun intended) :-)
Need to know a lot more details to know how effective and useable this will be and no doubt this will be explained further during WWDC 2021
Yes, according to the keynote we can deploy to the App Store right from your iPad