1, no index or index is not used (this is the most common query slow problem is the program design defects)
2, the I / O throughput of small and formed a bottleneck effect.
3, did not create computed column does not result in query optimization.
4, insufficient memory
5, the network is slow
6, check out the data too large (can use multiple queries, other ways to reduce the amount of data)
7, lock or deadlock (which is the most common query slow problem is the program design defects)
8, sp_lock, sp_who, activities of users to view, read and write due to compete for resources.
9, to return the unwanted rows and columns
10, the query is not good, not optimized