Run some of your frequently used queries though the query analyser and look at the execution plan. This will give you a break down of how much time is being used in each step of the query and should give an indication of where you should start looking to improve performance.
- long running queries
- locks and the duration thereof
Then you can start using the Query Analyzer. Add (nolock) where appropriate, index your tables and if you have ANY in-line SQL with variable where statements or more than one table then you should immediately move those to stored procedures.