Fast charging on iPad Pro is 14.5V@2A = 29W.
Fast charging on iPhone is 9V@3A = 18W.
Only the 29W charger supports 14.5V@2A = 29W and thus is the fastest for an iPad Pro.
But all of Apple's USB-C chargers (and, in theory, any USB Power Delivery-compliant charger) support 9V@3A = 18W and so none are faster or slower than others for iPhones.