I’ve experienced exactly the same problem over a number of years in Chichester Marina, not with barnacles but with tube worm, a coral-like creature which grows on propellers and saildrives, converting them into something resembling a loo-brush. Not very helpful as the growth clogs up the feathering mechanism on my Brunton’s prop and renders the blades more or less useless. I’ve tried all the commercially available “remedies” including the very expensive epoxy based ‘PellerClean’ (over £50 for a two-part pack), International Trilux out-drive, boot top and prop paint and various grease formulations, all of which were quite frankly next to useless. I’ve decided that the best solution, in Chichester at least, is to polish the prop to as high a finish as you can get over the winter lay-up and have the boat lifted mid-summer to have the prop and saildrive pressure washed mid-way through the season. I’ve just had my B32 Another Fantasy done, having experienced yet again the symptoms you describe i e low revs and poor boat speed. The most infuriating thing about all this is that although the prop resembled the aforementioned loobrush, the rest of the hull was completely free of any contamination, just a very thin coating of slime which was easily washed off.
I’m afraid you pays your money and make your choice. Good luck!
B32 Another Fantasy