Do weapon procs increase with level?
 

Let's say swarmcaller:

http://wiki.project1999.com/Swarmcaller

In the hands of a 60 ranger is it 50% slow or is it only 50% slow when cast by a sham?

I noticed that the proc on my velium weapon got a bit better while leveling but it stopped at 12 damage. I think it's just like a spell that can get better for a time. Most procs effects will prolly already be maxed at the level that they start to occur.

It works just like casting the spell would.

Decrease Attack Speed by 34% (L29) to 50% (L60)

So at level 46 when it starts proccing, it will be somewhere in the 40-45% range. At 60 it will be 50%.