The Domain Aggregate functions (DSum, DLookup, etc) are notoriously slow when used in queries. As NeoPa suggested, you should use their SQL Aggregate counterparts in your queries (SUM, etc).
If your sum needs no criteria other than those already in your query, you can add totals fields to your query very simply, as show in this tutorial:
Totals queries
If your sum needs additional criteria, you may need add a separate totals query and join to it, or write it out inline in your SQL as a sub query.
There are plenty of excellent tutorials out there... just look for "Access Totals Query Tutorials" or similar using your preferred search engine.
(We'd need to see your SQL to help with specifics)