Stock Vibrant? Same versions of Gmail?
I set it up that way after your post and I got exactly the opposite UNTIL I opened Gmail on my laptop (two phones, one desktop, one laptop). Suddenly the laptop got stuff and then the Nexus S, then the desktop, then the Vibrant. I shutdown the desktop and after a few more test emails it was back to the Vibrant, then Nexus S, then laptop. I think it's a bit more about connection management then some innate degradation of performance in Gmail running on the Nexus S. -Pk