Don't roll your shoulders, just go straight up as high as possible, then right back down.
As far as rep speed/cedence etc., it depends on your goals. Lifting slow does nothing for a strength athlete because all it does it teach your body to react slowly. However, lifting slowly can be effective if all you want to do is build muscle.
What I am saying is that for traps, I feel a slow cadence is not optimal. This is due to my own personal experience with olympic-style lifting as well as the people who I see training. In most commercial gyms there are very few people with traps, the ones who have them are usually athletes who do explosive lifting, a lot of deadlifts, or indirect work like strongman events. On the other hand, 98% of people in a gym are constantly shrugging, they are either rolling their shoulders all over the place or they are lifting VERY slow and only moving up about an inch or so, trying to lift fast with entirely too much weight on the bar and still only moving an inch. All of these people have no traps. In fact, most bodybuilders, even pros, have no traps. I have seen 150lb olympic lifters with beefier traps and necks that most 280lb pro bodybuilders, and I believe the reason for this is that traps just respond better to explosive movements.
I know I repeated my first post in a way, but to make myself clear, I think traps respond better to fast, explosive movements, straight up and down. If you do not know how to clean or do jump shrugs, then don't try unless you do. Most people in the gym, especially personal trainers, will also have no clue what they are unless you go to a college weight room or ask a strength and conditioning coach or someone along those lines.
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