You know, I gave you the benefit of the doubt and I actually pitted my S8+ fast charger against my iPhone 7 Plus on an iPad charger...guess what, charging from 20% to 45%, the S8+ got to 45% while the iPhone got to 42% at the same time. That's a 3% difference only. In the meantime, the iPhone did not even get warm, where as the S8+ was nice and toasty. Even if I extrapolate this to 100%, the S8+ will likely charge to 100% faster than the iPhone 7 Plus, but not by much, maybe a few minutes at best. The iPhone will likely be around 93%-95% charged by the time the S8+ is at 100%. Depending on how much the Samsung fast charging slows down the two might actually end up even closer. So there it is...the whole "iPhone charges slow" is pure crap. It does, if it is using the crappy stock charger. If you are using the iPad charger or another high-current charger, the iPhone can take the charge just fine and charges (in essence) just as fast as any top-end Samsung phone with Adaptive Fast Charge. If you want pictures, I can post those later too.
Edit: Just a point to note, both phones were not running any apps, had basically the same apps installed, same configuration, and both on Airplane mode. Basically, I equalized them as a user would under normal circumstances. The setup was the same because I use both phones and both need to be configured the same apps-wise.