I'm with SOTK on this, it's subjective.
Benchmarks between ROMs prove to be a highly inaccurate comparison so there isn't any way to quantify the speed.
That leaves everything on to user's perception of the ROM which can be determined by so many factors.
What kernel did they install with the ROM or was it stock?
How long did they run the ROM for?
What build were they running? Most of these ROMs being suggested have nightly versions and their performance can vary from version to version
What other settings did they tweak?
What was the user's expectation of the ROM going in?
How are they defining speed? Is it snappiness? Load time? Fluid game play?
My experience is that you need to run a ROM for several days to really get an idea of it's overall speed. Most ROMs are quick initially but after you get everything installed and start using it that's when you start to see slow down.
Additionally, the answer will never hold true for long. When the VZW Gnex came out there were like 3 ROMs. Overtime there will be more and they borrow tweaks from each other so that will close the gap between them over time.
I'm by no means knocking the question or the answers, they are all good. I just think it's important to help illustrate that the answers can't be clear, cut and dry. If you get one, I wouldn't put a whole lot of credit in it but i would check it out for yourself, especially if it's repeated a few times.
That being said, throwing another one in for Bugless Beast w/ Franco. It's a personal favorite, which is probably all most of the suggestions you are getting really are. No one picks a ROM because it's the slowest
