As your iPhone charges, it may get slightly warmer. In order to extend the lifespan of your battery, should the battery get too warm the iPhone's software will limit charging up to 80%. Once the iPhone cools down, the iPhone can be charged above 80%.
To speed this up, you may try moving your iPhone and charger to a cooler location. Please see full details here: https://support.apple.com/en-us/HT201569