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.
for all classes except rangers you want slow main and fast offhand for rangers with high PF/high dmg add and low cd spec its slightly better to have a slow offhand and a mainhand you can reach 1.5s swingspeed if you want high cd spec on ranger and charge a DA then you want to go with fast offhand slow mainhand too