What performance low-hanging-fruits do you know?

For example, I have developed an application that is deployed on numerous premises, and unfortunately, I don't have control over the SQL agent for all of them. However, I have implemented a mechanism in C# to assess index fragmentation and manage it accordingly, along with updating statistics.

I would greatly appreciate your input in the comments section regarding any other effective "hacks" that you know of, which can significantly improve performance.