SQL Query is getting progressively slower
Try SET ARITHABORT ON in your client application...

Why would SET ARITHABORT ON dramatically speed up a query?

I have almost always seen this to be differences between the ADO(usually OFF) and SSMS(ON) for the ARITHABORT setting. This setting can dramatically impact the Query Plan that SQL Server uses for certain queries. Here is a very detailed article (slow queries but SSMS works fine) and how to perform parameter sniffing troubleshooting to get to the bottom of it: http://www.sommarskog.se/query-plan-mysteries.html

One way to test this would be to set it OFF in SSMS (it is on by default in SSMS and off in client apps generally) and run the queries to see if they slow down:

From the MSDN article:


  The default ARITHABORT setting for SQL Server Management Studio is ON. Client applications setting ARITHABORT to OFF can receive different query plans making it difficult to troubleshoot poorly performing queries. That is, the same query can execute fast in management studio but slow in the application. When troubleshooting queries with Management Studio always match the client ARITHABORT setting.