Any OH weapon without stats gimps your main attacks. Dual wield white attacks do half damage and miss 24% of the time. The offhand doesn't get the benefit of special attacks which only have a 5% miss rate. This is why offhand procs or dps don't mean much.
Any weapon will proc poison at roughly the same rate, because the rate is normalized by a weapon speed modifier. Yes at a certain point, let's say if you have a 50% base proc chance, faster weapons will get you more procs because a slower modifier could put your proc chance past 100%. Crippling Poison has a 30% proc chance so even with the slowest offhand (2.8) you're looking at an 84% chance of applying Crippling. A slow weapon will actually give you a higher chance to apply poison on your first hit because your first attacks only take .5 seconds to cast but you'll be given the slower weapon modifier, increasing proc chance.
TLDR: Weapon speed matters far less than people think.