Disclaimer up front, we're Portal users, so we may not have the latest version of OD. The dashboard, that I published, performed so terribly that it was pretty much unusable through the JS app (although the Windows app was OK). Then I redid it through Web AppBuilder, and it works so great, and everyone is so much happier, that I feel I must ask this question: why does OD even exist?
Being as both the JS based web version of Dashboard and WAB are built on the same JS platform, I'm surprised you would have performance differences. Perhaps there is a configuration issue? Can you expand on the issue?