Dashboard performance is really going to depend on the client, since the browser is doing most of the work. Since the current Dashboards are on the 4.x JS API, even very large numbers of features will come in, albeit incrementally.
I guess what I'm saying is, I don't think there's going to be a clear answer to this. I can have a single layer with a single feature in it, but I could still write a popup expression on that feature to completely freeze up a browser. I've made dashboards with multiple maps and dozens of widgets with no discernible performance issues on my own machine. I've opened simple dashboards on low-end chromebooks and had to completely give up.
- Josh Carlson
Kendall County GIS
