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Many mid level Shimano rear der pulleys have a ceramic bushing in the guide pulley (a more descriptive term then jockey pulley) AND also have a small amount of end play to allow better pulley/cog centering and accommodate manufacturing tolerance and cable wear. Once the guide pulley's play becomes a rocking/canting slop (besides the end play that retains the pulley's vertical relationship) the shifting can degrade. Much like a lot of cable friction results in. The lower cost ders have a steel bushing and don't last as long. Both levels typically have a steel bushing (with minimal end play) for the tension pulley (the lower one). All these versions can have tooth profile wear over time and also bearing wear/clogging with grime build/mud up. It's surprising how much friction a chain der. system can attain if not cared for.
Of course with the tighter tolerances and smaller dimensions that a 10 speed system has (compared to 7,8,9) less system wear and/or inattentiveness results in greater problems and sooner too. Andy
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AndrewRStewart