This may occur if your table is not properly indexed. Since you are filtering
917 out of 300000 records, if the column "Source" that you are searching is not indexed, then the Database Engine had to search from the Start to End to find the matching values. Try adding a Non-Clustered Index on the column - If you already have one Clustered index on any other column on the same table.
Refer the below links for more details on Indexing
Understanding SQL Server Indexing - mssqltips.com
SQL Server Indexes Tutorial - SQL Server Planet
Indexes in MS SQL Server - Code Project