Well here's the can of worms.At risk of reopening a can of worms...
"Slow as a wet week" dates back to at least 1923 and has plenty examples over following decades.
"Slow as a wet wig" appears to be a local variant in some parts of Queensland based off a mishearing of the original saying and never made common usage.
(The original saying that was appearing more often from the 1890s and on was "slow as a wet Christmas"!)
My dad and his friends who never set foot out of Melbourne used to use the term slow as a wet wig to describe most of the players at Fitzroy just about every week back in the '60's. It was sometimes heard on the footy shows at that time as well.
It sort of intrigued me and taking the bravado to enquire what it meant I was politely told that wigs take a long time to dry. So they're really slow.
So it turns out that every time I've been hearing slow as a wet week which seems to be the more common vernacular these days is at least if not more valid historically.
And it's taken me all these years to find out.
Probably what I'll remember most from this wretched game.