I looked up 10s average speeds on GPS-Speedsurfing.com for UK spots West Kirby, top speed spot, Southend, Portland Harbour Weymouth and my local spot Poole Harbour.
WK just needs wind.
Southend needs a low Spring tide to expose a sandbar to get flat water
Portland needs a high Spring tide to work best, and 2020/2021 werent great years for wind.
Poole is more difficult to go fast, and people use small slalom boards rather than speed boards.
If the top speed sailors from 2006-2010 still sailed I reckon they would have gone quicker, although going fast depends mostly on conditions, wind, angle, tide etc.
If you look at an individual sailor like Simon Pettifer who gets new kit each year as he is top 5 UK slalom and is sponsored, his speeds have been improving over the years.
www.gps-speedsurfing.com/default.aspx?mnu=user&smnu=sessions&uid=4276&StartRow=0The R is for still regularly sails at the spot.