There is some performance hit with feature layers for polylines and polygon features (Silverlight API 2.4).
I implemented MaxAllowableOffset and that increased performance by 7 fold. But still delay of 10 seconds or so for some of the feature layers (with only small fraction of production data).
I am using OnDemand mode with OnDemandCacheSize of 1000 (I've tried 0 to 50 to 1000).
For MaxAllowableOffset using: MapResolution * factor, where factor ranged from 2 to 2000.
Are there other performance optimization techniques available...?