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Actually one of us runs a 4s and has upgraded to iOS7, they haven't reported any problems and when I asked them just now if they're happy with the speed they said they hadn't noticed a difference to iOS6.
The iPhone 4s was the first iPhone with a dual-core processor, and was much much faster than the iPhone 4 which can also run iOS7, so I think it will be fine. Apple are very quick to cut off hardware from an upgrade if the experience isn't going to be up to scratch, as we saw when the original iPad was left out of iOS6 even though iPhone 3G (an older device) got it. As such I'm happy to trust them that it'll be fine, but as ever you should verify this if in doubt!