The query pool refers to Formula Engine activity for queries. If you are seeing consistently high queue lengths, but not high CPU utilization, you many want to adjust your MaxThreads and/or CoordinatorExecutionMode properties for your SSAS instance.
More details on these settings are covered in section 6.11 of the SSAS 2008 Performance Guide.
Remember that the FE is single threaded, so increasing the query pool setting may not improve performance of any one query, but may improve the performance in handling multiple simultaneous requests.