How fast it would run depends on how you create your menu button. If you're using a sprite as your 2d image (as you say in the thread title) you can probably draw about a hundred each sync with no ill effects. I tried 1000 once (by accident) and the screen went incredibly jerky, but I have had at most 60 sprites running successfully with the Fps remaining a steady 55-60 (with Sync 0).
If (as your post suggests) you use the "Box" command, then you'll get a noticeable frame drop after just five calls. Your best bet is to stick with sprites - 2s in 3d is quite fast in computer.
Of course, it you want menu buttons on a 3d backdrop, couldn't you just use 3d plains and "Lock Object On"?
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