Thanks for the reply. Adding series before the axes is indeed faster but still pretty slow compared to using just one series. Once you get up to about 50 series (with just a few points in each series) there's a noticeable second or two pause on my S5 before the graph appears and the thread continues. This is better than the several second pause if you add the axes first, but still not usable, so I'm going back to using just one series.
For other people looking for a workaround, I had some success with adding one series to the graph every .1 seconds or so using a Handler#postDelayed() loop. You could also try adding the series on a separate thread and show a spinner above the graph, but it may have to be done on the main thread.