Apple announced iOS 11 in the summer of 2017, making usual updates to several parts of the OS and their browser of course. Of the several updates that have gone in to Safari 11, the needed Resource Timings was one of them.
But as we all know iOS has a signficant footprint in North America with a near 50% market share, and just about half of that in Europe. So adoption of new OS releases can have significant impact to users and features available to developers. So it goes w/o saying that a 6% adoption in 2 days is significant - which would put them @ 75% adoption in just under two weeks.
Now, to parse a touch more, let's just say that only 3/4 of the 10.x users will/can update to 11, that still brings us to about 60% adoption in same time period. Not bad. You can read more about it at Stat Counter.