From what i have read in previous posts (the one i'm thinking of is what cyzaine posted a long while back) is that slow is mitigated from the get go, by half of the mobs level that you are casting it on. so a 50% slow casted on a level 50 mob, slows the mobs attack speed by 25%.
so to actually be of any use while duoing, you would likely need a slow clickie that does a 40% slow or more, and i don't know of any that are that high, just thought you should know about slow mitigation before buying a clickie of some kind.
Lowdown on slow mitigation: Mobs have a minimum slow mitigation of (level / 2). Some mobs will have higher than that or even total immunity to slow. If a mob has a slow mitigation of 50, this means that any slow affecting them is reduced by 50%. So let's take the example of a regular level 50 mob with 25 slow mitigation and Turgur's Insects + shaman mask focus
Turgur's Insects: 50% slow + 5% shaman mask: 55% slow. The mob mitigates 25% of the 55%, bringing the slow down to ~41%. The mob will attack at ~59% its regular attack speed, instead of the 45% it would if it had no slow mitigation.
Slow mitigation is there to keep the gap between slowed and unslowed mobs from growing too wide as it would on live where a mob would literally do 5x the dps unslowed.