Ten minutes of investigation yields…
To try to be really careful and controlling about this experiment, I closed Chrome, then re-opened it, and closed the window (zero tabs). Then I openend tabs one at a time, recording measurements from Activity Monitor (after they settled down) in Fathom. MacOS 10.6.8, Chrome 19.0.1084.54. MacBook Pro, not new.
Results do not match the Mythbusters “testing” post referred to by Shawn here. Of course, the original test opened some “top 40” sites list for their 40 tabs. But this gives some interesting insight into what may be going on.
The number of threads was praactically constant at 26 or 27.