What are the full specs of your Mac?
Doesn't DX9 mean DirectX 9, which is a windows thing?
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Not sure which specs you need: MacBook Air, Intel, 2.13GHz dual core, 2GB memory, NVIDIA GeoForce 9400M display card.
Can you try doing all this, just in case:
If the performance is still choppy after that, then I wonder if maybe it's a memory issue. 2Gb RAM is not much nowadays, so your Mac could be doing a lot of paging, which is going to affect Unity's performance. Have you tried a reboot and shutting everything else down in the background to see if it behaves better?