How to delete a Database -Error throws the action cant be completed
You can't delete it because it's open in SQL Server. 

If you want to delete it, you'll have to stop SQL Server and delete the files.

You could also go on the server and run  DROP DATABASE WSS_UsageApplication;, assuming you have permission, and the database isn't in use. You may need to run ALTER DATABASE WSS_UsageApplication SET SINGLE_USER WITH ROLLBACK IMMEDIATE; first.

But really, deleting a 62 GB database because your 'application is slow' seems like a terrible idea. Since you're here asking why you can't delete a database that's in use by SQL Server, you probably haven't taken steps to tune anything. SharePoint is supported by Microsoft. Have you opened a ticket with them?

62 GB is also absolutely nothing. I'm typing to you from a laptop with two 1 TB SSDs in it. Is this really a concern?

You should at very least take a FULL backup of the database before proceeding.