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No real mystery (as far as I am aware). If we are talking about aerofoils (in this case the MRBs) then reduced performance is almost always symptomatic of 'dirty' airflows caused by changes to the shape of the aerofoil; usually either by damage (eg: hailstones) or accumulated properties such as frozen rain and perhaps even your Swiss pollen - wouldn't know about the latter because I've never really had pollen-related rotor issues!
Rain (unless frozen or getting close to being frozen) does not interrupt the shape of the aerofoil but disperses on impact or passes around the blade (often as 'mist' as a result of being partially vaporised by the surrounding air pressure). It also depends on the intemsity of the rain!
Sav