There was a video posted in one of the other threads that shows the differences quite cleary along with multiple other threads discussing how bad this is.

However as Aphel pointed out, fair birds in rain = perma heavy until the rain lifts. It does make a huge difference and is totally out of balance right now. Get up to the higher stats/pedigrees and race with a fair bird and you will see the issues. R40-80 are pretty much the starter races for any higher pedigree bird and so you won't notice how bad it gets.