I went from an iPad 3 to a Pro 9.7 and it was a hugely upgrade and she will definitely notice. My iPad 3 was so laggy it was unusable, but the new iPad flies.
I'll be more careful about accepting iOS updates from here on out.
Uh..."unusable"...??? Maybe stretching the comment. But "yes" you will notice a big difference of course with anything newer, but the iPad 3 is still useable. It is heavy, "yes" compared to the new slick silicone casing verses the old aluminum design. It is manageable.
As far as upgrading to iOS 10...probably not of course. IOS 9 is probably the last for this old horse I think and then may be totally laggy. I have used it daily since 2012 (and even writing this post). Did we who bought it when it came out in March 2012 get ripped off? "yes". Especially when they came out with the iPad4...six months later (still peeved thing about it). But...it has been reliable regardless and got my money's worth out of it. I wanted to get a new iPad for my wife (also has a iPad 3). She said, "why, it still works!" Amazing.
Now..I have an iPad 1 that is barely usable. Just for the kids to play a few games and some basic stuff, probably the iPad 2 is getting unusable too, put the iPad 3 is still going. Will I upgrade, absolutely, hopefully soon.