Solar panel is 10W output so in one hour is 10Wh. The iPhone 6S It has a 6.91 Wh battery, the pre-trickle 80% period is so 5.5Wh for 80%. so you'd expect 0%-80% in 5.5/10 = 0.55hr or 33mins, or 1% in 33m/80=0.41m or 25s , so at 10W output (max possible from that panel) a full recharge should afford 1% every 25s.
You got 4% in 5m, so 1% in 1.25s so you were observing 3x slower than theoretical.
- the iPhone s6 is a slower recharging phone, from reviews seems that may be the case
Observe it draws 6W, and only as much for half the time, so the pre-trickle phase ends at 0.9 hours not 0.55hr.
- the act of using a phone slows its recharge, if you had turned it off, it might have charged faster. 5 mins is also a short time, the phone still warm.
Note: from any % to 100% is utter nonsense to ever measure as trickle charge makes everything move at a crawl, only a % to 80% is of any value to compare.
Another way to put this is that the Anker 21W solar panel is "too powerful" for the iPhone, so your review method to fully use all the output of the 21W via the Powercore II 20000, is one approach. Another approach is to use the 2 USB outputs to charge the iPhone plus another device to not waste sunshine, or (what backpackers seem to favor) use the 14W version to shave a few oz.