Using the official 25W charger and USB-C-to-C cable from Samsung, my Galaxy S21 Ultra's 5000 mAh battery can charge to more than 50% in 30 min's.
Thank you all for the feedback. I definitely wouldn't expect it to charge as fast compared to the official 25W....but my charging time seems like a pretty outrageous difference. With the battery issues reported I was wondering if mine is defective or not working well.
I am guessing that you had not enabled Fast Charge when setting your phone up, in...
Settings > Battery and device care > Battery >More Battery settings > Charging... Fast charging/Super fast charging/Fast wireless charging