Fast weapons should be fine at your level but if you read the tooltips for each of your attacks you will notice that stormstrike, lava lash, and windfury are all based on weapon damage. Slower weapons have higher damage so if you use slower weapons with a larger damage base, your main attacks with hit harder. Also when you start using flametongue on your offhand weapon, you will notice that the damage procced off of each swing is higher when using a slower weapon. Therefore a 2.6-2.8 speed weapon is optimal to fill this slot.
Also using a slower weapon in your offhand will avoid it taking away flurry charges and it will not take away potential windfury procs if you have wf imbued on both weapons. WF has hidden 3 second CD. Also, when you equip a weapon in your off hand, the damage is cut in half. So if your main and offhand weapons are the same (both have the same damage), your off hand will deal on average half of the damage your main hand weapon does. This is the reason you do not want WF to proc off of your off hand, because if it does then the damage will be at most half of the damage that you would deal with a proc off your main hand. Don't worry about not dealing enough white damage with slower weapons. You should be critting constantly so flurry should always be up and with Winfury Totem down you can get a 2.6 speed weapon down to about 2.0 speed fairly easily. Berzerking is nice too if you are a troll and it has a relatively short CD. Sorry for the wall of text but there are just so many benefits to using slow weapons over fast weapons as an enhancement shaman.