It's definitely made my iPad 3 much slower (and I think it shares the same processor as the iPad mini). A bit frustrating, but that's the march of technological progress I suppose.
It is but it usually isn't so noticeable with a last gen device. Then again as you stated it is running the A5 which is a 2 or 3 year old processor.
I keep seeing that it's made the iPad 3 slower but I haven't noticed anything with mine. Then again I keep my iPad 3 pretty bare with media and games so maybe that's what is helping.
Not exactly the same CPU. The 3rd gen iPad uses A5X while the iPad Mini uses the A5. Then again, the X part with the beefier GPU is mainly to handle the extra pixels (4X the pixels) in the full size iPad. The only problem is that drawing off screen is CPU bound so very slow in 3rd gen iPad compared to iPad mini or iPad 2. That is the main complaint I have heard about 3rd gen iPad (which I also have but haven't upgraded to iOS7 yet).
Regarding slowness in older devices, I have read that doing a fresh install (or reset) may help although that may be more trouble than its worth.