The Microsoft SQL Server 2008 R2 query optimizer fails to choose the spatial index despite the index's beneficial effect on query performance.
Usually, this issue occurs on a multiprocessor system that is lightly loaded. The problem is less likely to occur on a heavily loaded system. It has not been seen on a system with only a few processor cores.
The problem is a bug in Microsoft SQL Server's query optimizer: Microsoft Support - FIX: Slow performance when running a parallel query in SQL Server 2008 R2
After applying the fix, enable SQL Server traceflag 4199 globally. Use the dbcc command:
DBCC TRACEON(4199, -1)