scheduled delete performance cost

What would be the performance cost if I deleted all the rows from a table in MSSQL every 5 minutes?

I'm doing this to keep track of whos online. Every time a user goes to a new page they get logged into a table called WhosOnline. Every 5 minutes the log will get deleted.

What would be the performance cost of this and if its high, does anyone know a better way to keep track of whos Online?Forget it, I figured out how to do it. SessionID & UserName in a database.