Question about Samsung Eb-Pg935

The battery probably needs around 6h 30min to fully charge (if it has been completely emptied).
10200mAh is more than 3 times the capacity of a mobile phone battery, which justifies the time.

10,000 mAh is relatively large and this powerbank only has USB, so that is quite possible.
The question is what was used to charge it?
I have one of the same size from Energizer, but mine has a power supply unit (as used for laptops) with a standard type J plug (SEV 1011) for the socket. I use it to charge at 19V and up to 3.5 A, but it still takes about two hours to fully charge.
With USB, of course, it depends on what you are charging with.
A standard USB connection has 5V and just 0.5 or 0.9A. If it's a battery charging connection, it's 1.5A.
However, it is better to use the charger of an existing smartphone or tablet, as these have a current of between 1 and 2A (usually more like 1.5 to 2A) and therefore charge faster than a computer connection.
Using a device that supports Quickcharge (Adaptive Fast Charge is just a Samsung marketing name for the same technology) increases the voltage from 5 to 9V and charges about 50% faster.
Still, even with Quickcharge, I would allow at least 4h, without it more like 6h, for a full charge.

The powerbank can be charged with a Samsung quick charger. My powerbank was fully charged for the first time after about 3 hours.