Hi. Visakh16 helped at length in getting this report to work at https://social.msdn.microsoft.com/Forums/en-US/a83aaf7c-2689-40b2-8d63-e55789b3d374/data-set-runs-twice-instead-of-once?forum=sqlreportingservices . Specifically with what ssrs does and doesn't do with datasets.
I created a dumbed down version of the report for purposes of that post and am re posting 3 of the images here. In reality the tablix's in the sub report are more complicated and very different from each other. But the bottom line is that the master report has a tablix driven by salesmen, each cell calling the subreport, and the subreport produces 8 tablix's that summarize each salesman's performance.
I spent yesterday tuning the db engine side of things and got huge performance improvement (down to about 2-3 seconds from about 10+ seconds).
Now I'm focused on the visual (ssrs) side of things which I would have thought should perform better than it is. For an overall total of about 625 records across 6 salesmen, ssrs performs pretty badly. Maybe half the sub report's tablix's have more than 2 but no more than 4 rowgroups. The report takes 25 seconds to render. I don't see much of an alternative to using a subreport for this kind of thing but wonder what the community thinks about both tuning and alternate report designs.