Thank you for bringing this issue to our attention via the community.
We regret to hear that this PowerCore 20100 battery doesn't fast charge your S7 anymore and so sorry for any inconvenience caused to you.
Please note that your S7 is quick charge enabled, to maximize the charging speed, you need a battery which supports the same technology. However, this PowerCore 20100 battery doesn't support quick charge technology so the charging time is longer than your OEM wall charger.
To confirm the issue, As suggested by @elmo41683, could you please use another working cable to see if there is any improvement? Please also inform us how long it takes to fully charge your phone at standby mode from this battery. Per our internal test, it takes 1 hour and 53 minutes to fully charge your S7 from this battery.
If the battery still charges your phone slowly, you can contact us via firstname.lastname@example.org and we will be more than happy to help you out as per the 18-month warranty policy.
Sorry for the inconvenience caused and thanks for your understanding.