So you have basically right now a limiting factor more of electronics than you do of lithium ion or lithium polymer chemistry. As Anker don't make the cells they design the chips and housing, it is Anker who is the limiting factor.
Take example of a 18650 good quality lithium ion cell. It would have say 3400mah and be capable of being recharged at 1c so 3.4A / h and discharge at 2c so 6.8A/h, at a medium voltage of 3.7 you get a recharge rate of 12W and a discharge rate of 25W. The slower you recharge in particular the long the cell lasts, so there's a trade-off between speed of product and warranty costs. So the cells themselves say a 4 cell 13600mah powerbank could recharge in 1 hour ingesting at 50Watts and actually have the lithium discharge capabity of 100W so enough to charge the highest USB-PD profiles..
The voltage of the cell increases as you recharge so the buck converter which steps down the 5V input has to regulate its output slowly upwards, that is where the technological limiting factor kicks in as you're incapable of having perfect electronics efficiency, a good design would be 95% efficient so in the case of just 1 cell you'd have 0.6W of loss in heat, or in say a 4 cell 13600 mAh system even though the cells could recharge in 1 hour you'd have heat of 2.5W of heat or over an hour is 2.5Wh or 9058 Joules. Heat into temperature is material specific (specific heat density) but say 1 gram of copper would raise its temperature by 1C needs 0.385 of Joules so that 9058 Joules would raise 1 gram of copper by 23527C. Obviously not as that heat is dumped in this case over an hour and it radiate out a lot. The melting point of copper is 1085C, so excluding radiation and conduction, to avoid the copper melting you'd need at least 21g of copper as a heatsink which is 8.96g/cm3 so need 2.4 cm3 of copper and so if you made it really thin to dissipate heat say 1mm thick then is 24cm2 or 5cm square.
This then becomes impractical in a pocketable device.
So there is a limit, related to heat of recharge. As electronics gets more efficient then you have less heat from the electronics so for given mAh you can recharge faster and discharge faster. Given Anker is aiming for a price usually around the $20-$30 mark, this limits the technology you can use.
So this is significantly why powerbanks are getting faster, because the electronics are improving, or faster for a given cost.
With the slap shape of the Powercore II it is more heat-dissipating friendly, the surface of the Powercore II 10000 is 44cm2, compare with say the Powercore 10000 is 35cm, so there's 25% more surface area (just on the flat sides).
The IQ2 likely uses newer smaller manufacturing techniques, they should be more efficient.
So you then have where Anker's know-how kicks in, they know for the materials they use the specific heat and conductance and know if they increase their recharge speed in particular what is the safe upper limit.
The surprise is the shape should allow a safer upper limit, and newer electronics should cause less heat per unit of power and so allow to run faster to allow the release more power. So if you could make say the 5000 Slim recharge in 3 hours and it doesn't get hot, I'm surprised a Powercore released July 2016, in May 2017 you're 4 hours recharge for a physically larger device.
I guess Anker knows best.