Might be moisture on some parts, subtly altering the aerodynamics.
Thats where my money would be, same reason Gliders dont like flying in rain, airflow no longer laminer over the wing. Droplets and water runs will create lumps and bumps = more skin friction / parasitic drag.....might only be subtle changes but its the same reason we clear ice off our wings...(though ice is obviously more serious)
Poses some interesting points to think about, a wet airframe will be heavier than a dry one with the wings now less efficient and creating more drag for the same speed and AOA as when dry so in theory there should be a performance loss flying in rain....anyone actually noticed a difference in a spam can ? Obviously glider drviers will have but cant say I've ever really noticed in a Cessna and given I live in Scotland I've spent a lot of time flying in rain....!