I see Isle SUP has conducted a speed test between inflatable and epoxy boards as published on SupConnect:

www.supconnect.com/sup-gear/inflatable-vs-hard-paddle-boards-a-speed-comparison

The upshot of it is they compared a 12'6" x 31" inflatable with an epoxy board of matching dimensions over a half mile flat water course and concluded that the inflatable boards was a little over 5% slower.

Important to note that the 31" width and the fact that the Isle epoxy board weighs in at almost 18kg (WTF?) suggests we are not exactly dealing with latest race board design principles here.

Might be good to compare 2016 release Starboard / Fanatic / Naish carbon race boards used at Battle of the Paddle with those companies' best inflatable race boards as soon as they become available.

A flato is a flato, and a heavy epoxy is a heavy epoxy. It's a bit like comparing the speed of my 14 SIC bullet with a Starboard 12'6 X 23" wide Sprint over a 5.2 metre course in my above ground pool, on a choppy day....

So only 5%....so many variables come to mind