I have never tried any perf testing on number of items in a list box, for example. I'm sure there's an optimal limit, but that will also depend on how the control is used. For example, a single 1000-item list box will not have the same effect as a 1000-item list box in a repeating table with, say 1000-rows.
The over all number of controls and complexity of the view all play factors in performance.
I don't believe there is any one answer to your question, but rather that it will be specific to your form, etc.
Greg Collins [InfoPath MVP]
InfoPath Dev