Performance is most often related to the code or the structure. But, sometimes it can be other things. For example, do you have any process for maintaining statistics on your servers? If not, you might want to, at a minimum, run sp_updatestats to get the statistics updated. But performance tuning is a giant topic. You need to gather metrics, compare them, comprehend their meaning and then begin to address the issues they suggest. If you're just getting started in SQL Server, you're better off focusing on the basics like ensuring that you have good backups in place on your databases, things like that.