I got a report of a relatively simple WinForms app created by a senior (!!) developer who left just as it was released taking 3 minutes to load.

Step through it.. Narrow it down to one stored procedure.

Open said query, every join is a left join.

None needed to be a left join.

Change them all to inners, app now loads in 5 seconds.

Left Joins: For when people can't be assed to learn SQL basics.