Many people champion fast offhands but testing that has been done has shown that it doesn't catch up to Left Axe damage until about 18-20 consistent swings. I champion a primarily slow offhand. That being said, it can get more complicated, especially considering mainhand speeds and growth rates.
Here's how I run all of my dual wielding characters;
fast mainhand/slow offhand - for landing the initial style in a chain with a low to medium growth rates
slow mainhand/slow offhand - used up in follow-up styles that have medium to high growth rates
One exception: on my Hib characters ONLY for the side chain (Icestorm etc.) I use the slow/slow for the first hit and slow/slow for the follow-ups because the initial growth rate on Ice Storm is better than most other positional styles.
The Drunken Ranger!-keeping Devs honest since 2012
Also playing a dual-wielding Saracen
The Drunken Ranger’s Guide to Drunken Rangery