Hey Susan -
It's not out of the ordinary to see a slight hit in performance when using Data Server as it adds another "layer" that your data needs to go through:
Database > Data Server on Tableau > VizQL Process on Tableau Server which renders your report
Database > VizQL Process on Tableau Server which renders your report
I recall one of our fine support folks mentioning that version 7.0.11 contains some fixes that address problems around caching and metadata about data in Data Server...so you also might want to make sure you're on the latest/greatest.