For lots of data? How much is "lots" ? And how long is "long time" ?
Yes, a DSum is slower than a Sum but not necessarily by a huge lot. But... how many columns are you summing? If you did sequential DSum calls on multiple columns of the same table, you would do better to write one Summation query and then open that query to retrieve its individually summed fields.
The alternative to DSum is Sum as an SQL Aggregate, which are probably THE fastest methods available for summation in Access because the query engine code is compiled. (Whereas using recordsets is interpreted.)