Well, this turned out to be a bit interesting. It turns out that when you really pare everything down to the bare bones, so that you are only testing the time it takes to pass a JSON object in or out of a user defined function, the way you pass it makes a very big difference (at least for large objects). The results are below, and it is just as undeniably clearcut as I could have hoped for.
Passing/returning a JSON Object by name and using eval to copy the value into a variable has essentially the same cost in time regardless of how large the JSON Object becomes. On the other hand, passing/returning the object itself to/from a user defined function slows down tremendously for large objects.
This only really applies for large JSON Objects though, for small JSON Objects it really doesn't seem to matter. For arrays smaller than size 256/512, there really wasn't a meaningful difference in the time it took.
Pass by Value
203, 188, 187, 204, 187, 203, 250, 281, 375, 578, 985, 1813, 3578, 6953, 13750, 27719
187, 187, 188, 187, 203, 219, 234, 297, 391, 578, 953, 1782, 3563, 6921, 13766, 27750
187, 188, 172, 187, 188, 204, 234, 281, 375, 594, 968, 1766, 3547, 6921, 13687, 27547
187, 188, 187, 188, 203, 219, 234, 281, 391, 593, 969, 1782, 3563, 6937, 13750, 27563
Pass by Name
203, 203, 203, 203, 204, 188, 203, 204, 203, 203, 203, 187, 203, 203, 203, 203
203, 203, 203, 188, 203, 203, 204, 203, 187, 187, 187, 203, 203, 203, 203, 203
188, 203, 203, 203, 203, 203, 188, 188, 203, 203, 203, 187, 187, 204, 187, 204
203, 204, 203, 203, 204, 187, 187, 187, 187, 188, 188, 188, 188, 188, 203, 203
Return by Value
297, 297, 297, 297, 297, 297, 328, 375, 438, 609, 922, 1563, 2829, 5390, 10937, 23500
297, 281, 297, 281, 297, 312, 313, 360, 453, 594, 907, 1563, 2828, 5360, 10765, 23641
297, 281, 281, 281, 297, 297, 313, 375, 437, 594, 907, 1563, 2829, 5375, 10766, 23484
296, 282, 282, 281, 297, 312, 313, 360, 453, 594, 922, 1563, 2828, 5390, 10922, 23531
Return by Name
312, 329, 312, 313, 312, 313, 312, 313, 296, 297, 296, 297, 297, 297, 312, 313
297, 312, 313, 312, 313, 297, 312, 313, 297, 312, 313, 297, 312, 312, 312, 297
297, 313, 312, 313, 312, 313, 312, 297, 313, 312, 313, 312, 312, 312, 312, 312
313, 312, 313, 312, 297, 313, 312, 297, 297, 313, 312, 297, 297, 297, 312, 312