let's put it this way,
iPad 3 on the original iOS was slow to begin with.
it's basically an iPad 2 with (4?) times the pixels to push with basically the same chipset.
now, with iOS 9, the device is very very slow. it shouldn't have been supported for this iOS to begin with.
for context, i have around 60k images or videos in my photos libarary.
to sync with iCloud, it took me 3 weeks and it's still not finished syncing lol
bottomline: if you have this legendary (yes!) device, try not to update to iOS 9 if possible, and don't use iCloud features. basically you can only use it for safari (barely) and watching videos downloaded into the device, and looking at images (locally). and that's it.
p.s. it is still legendary because it is the first retina display iPad, and the largest battery, and probably the physically largest chipset ever put into an iPad, thus the most 'pushing the boundary' iPad apple has ever put out.
but it was very early in the device development cycle so the succeeding devices advanced so much quicker, and thus leaving this iPad 3 as a museum relic.