Impossible to say without more information. It could be because your indexes are big. It could be that your tempdb is too small to support the sort operations, so there's a delay while it cycles information in & out of disk. It could be you don't have enough memory to do more of the sort operations in memory. It could be that you have contention on resources from queries or whatever running at the same time. There are probably more things I'm not thinking of. Run a query against sys.dm_exec_requests to see which processes are currently blocked or blocking. You can also see what processes are currently waiting on in terms of resources.