Roman's Weekly SQL Server Tip - Don't use compression with SQL Server files
In this tip I may be stating the obvious, but in the last few years and recently again I've seen a few SQL Server installations where someone was trying to save space and in doing so they enabled Windows compression on the folders where the data and log files were residing. This was usually done by system or network administrators that were not so familiar with the SQL Server side.
This setup is a BAD idea and it results in queries and updates taking a few times longer than they would if the files were not compressed. Believe it or not, this issue often goes unnoticed for a long time. If you administer or examine SQL Server for your customers and clients like I do, I recommend you add on your checklist to examine all folders used by SQL Server and make sure compression is not used.