Performance of requests in system:completed_requests

when I query system:completed_requests I can see a lot of queries that take more than 1 or 2 seconds however, when I try to see there plans to understand what is the problem I can get an answer in less than 0.5 a second in 90% of the times, how come most of the slow queries I have run a lot faster when I try to execute them form the query workbench?

may be load on the system. Post one of the query plan. Did use query parameters ?