I don't have an iPhone 8 so I only have several years of experience wireless charging Samsung phones. Wireless charging will always be slower then plugging a cord in. Both my S8+ and Note 8 take about 2.5 hrs to charge on my Samsung Fast Charge stand. This is going from 5% to 100%.
My Note 5 would take about 90 minutes, my S7 Edge would take about 2 hrs, all starting at 5% battery.
Plugging a cable in, the S8 and Note 5 take about 90 minutes from 5%. The Note 5, 65 minutes and the s& Edge, about 80 minutes.