If your power bank is charging slowly, it's always best to first try using a different cable and wall/car charger to see if this fixes the issue.
Naturally, power banks with large battery sizes are going to take longer to charge. If your power bank is charging slowly it could be due to using a wall charger with lower AMPs/power.
Wall chargers come with different AMP levels, and usually this is printed on the charger itself. 1AMP wall chargers are the slowest wall chargers available - it can take over 24 hours to charge a power bank 20,000mAh or higher with one of these.
So if you're using a 1AMP wall charger, this is likely to be the cause of your power bank charging slowly.
If you'd like to achieve faster recharge speeds, upgrade to a 2AMP wall charger or even consider a wall charger with 'power delivery'.
To find out more about fast charging your power bank (and mobile devices), head to How to fast charge your device.
For a full list of recharge speeds, head to How long should it take to charge my power bank?