Troubleshooting SQL Server Performance
Whenever the performance of queries against SQL Server decreases, check on the following:
If you transaction log file is enormous, it may be that the recovery model of the database is set to FULL, which means that it requires regular backups in order to clean up past transactions.
You can confirm this is in the properties of the DB itself.
The most urgent step is to perform a full Backup of the DB.
Run the following statement against the database.