Grantmac said..
I'm sure that 15kts is a rough average speed for many foilers and likely to be possible for many wingers.
My sessions on Slingshot 76/84/99 wings will generally be slower - around 12 knots average when windfoiling, with maybe a knot more for the 76 and a knot less for the 99. Here's one of my typical sessions with the i84:

This one was well powered (18 mph gusting to 28). Light wind sessions are slower. Top 2 second speed was 16.1 knots. This is with a highly accurate GPS. Many GPS watches tend to "exaggerate" top speed by a few knots on a regular basis.
For winging, my wife's speed on the i84 are typically a couple of knots lower in the same conditions.
Other foils are faster, though. In this tutorial, Sam Ross easily reaches mid-20 knots, with few white caps to be seen:
Not sure what foil Sam Ross is using there, but Swindy just posted a 26.7 kn session using Starboard GTR and Supercruiser foils:
gpsteamchallenge.com.au/sailor_session/show?date=2020-11-26&team=2