I have done some testing with the 3rd party iPhone chargers recently. I have a device that measures the number of watts the charger is generating for the phone. The limit on wireless charging is 7.5 watts and many of them don’t quite get there. Most are between 5 and 6 watts. Now this is about the same as the stock Apple charger that comes with your iPhone. I ended up buying a 3rd party charger that supports up to 20 watts for the fast charging compatible iPhones such as 8, X and Xs. You also need the $20 Apple usb-c to lightning cable, but the result is a truly fast charge. Of course my 2015 Samsung Galaxy s6 came with a fast charger in the box, but I am glad that with pricy accessories we can have the same with the iPhone