I don't think there is any suggestion that the Slow Zone actually limits human intelligence - for example the human characters in Deepness in the Sky didn't all

I don't think there is any suggestion that the Slow Zone actually limits human intelligence - for example the human characters in Deepness in the Sky didn't all become super-intelligent because they entered a artificial bubble of the Transcend. Of course, when they descend into the Unthinking Depths there are problems.