Simple count of points within a radius takes too long

Question

I have reduced my table to just two columns - an ID and a Geography column. I have a spatial index on the latter and a clustered PK index on the former. There are 10m records in the table. The indexes are not fragmented.
A query to get a count all points within 1000km of a specified point returns around 7.5m but takes 30+ seconds to execute.
The query plan shows use of the spatial index with cost 9% then clustered index seek on my primary key taking 83%.
I have tested STDistance and STBuffer - the latter is slightly faster.
SELECT COUNT(1) FROM testdoserate2 WHERE ([Location].STDistance('POINT (16.373813 48.230391)')) < 1000000
declare @referencepoint Geography = 'POINT (16.373813 48.230391)'
declare @radius Geography = @referencepoint.STBuffer(1000000);
SELECT count(1) FROM testdoserate2 WITH (nolock, INDEX(IDX_Location))
WHERE Location.Filter(@radius) = 1
Is there anything else I can possibly do to speed this up?

Answer 1

Have a look at this thread, especially the info on custom tessellations and the "optimizing point queries" whitepaper:
https://social.technet.microsoft.com/Forums/sqlserver/en-US/3bbdc511-2a08-4075-b989-d98b70d0bedd/sql-server-express-performance-limitations-with-ogc-methods-on-geometry-instances?forum=sqlspatial
In addition: If your query returns a lot of rows (or a large count of them) and/or if your query and your spatial index doesn't eliminate most of the candidate rows, you can have perf problems. 1000km radius does sound like it could produce a lot of
rows, which cause a lot a seeks on the base table in your query plan (capture the *actual* plan and look at the "Clustered Index Seek" iterator right above-right of the "Filter" iterator). 
Look into tuning the parameters of your spatial index using sp_help_spatial_geography_index. You're looking for largest Percentage/Number_Of_Rows_Selected_By_Primary_Filter and  Pecentage/Number_Of_Rows_Selected_By_Secondary_Filter, which the default
spatial index parameters don't always give you.
Hope this helps, Bob