VERY vast topic . one can spend months or even years on it.

a lots of thing could be covered here like

query redesign

index redesign ot missing indexes

unneccassry order by or select columns.

unwanted intermediate data manipulation.

off execution plan or outdated statistics.

fight between table variable or temp table

usage of tvf function.

missing or imporper filters like where or and clause.

better google these terms 🙂