According to The Verge[1]:
> iPhoto was often criticized for choking on large image libraries, but Apple says it built Photos to handle large and growing photo libraries, since people are taking so many more photos than they used to before. From our experience, it seems that Apple's efforts have paid off: the new Photos app effortlessly scrolls through thousands of photos, [...]
[1] http://www.theverge.com/2015/2/5/7979925/apple-photos-app-ne...
Photos will be replacing iPhoto and Aperture: http://www.macrumors.com/2014/06/27/aperture-development-sto...
I believe Adobe has already released a plugin to import your Aperture library into Lightroom.
Unfortunately Aperture is also being discontinued...
I'm just missing some statement on how the performance is, especially since we all have like several hundred GBs or even TBs worth of pictures nowadays. It's only a preview with screenshots though...