If you see this warning when you hit publish, don't panic! We've implemented this warning system as we've found that when a showcase gets to 500+ slides, the performance can be negatively impacted for the end user.
If viewing on the tablet app, particularly on a slower device, it takes much longer to download; if viewing on the browser, particularly with a slow network speed, it takes much longer to render every slide. So we wanted to provide a guideline as to the 'ideal' showcase size that would save folks from getting too frustrated with the performance.
There is not yet a point at which the system will forcibly prevent you from publishing a showcase.