Huh, will you look at that:
$ time find s04_epi -type f |sed 1q s04_epi/s04_epi.0001
real 0m0.096s user 0m0.002s sys 0m0.014s
$ time tar tf s04_epi.tar.gz |sed 1q s04_epi/s04_epi.0001
real 0m0.016s user 0m0.001s sys 0m0.008s
Thanks! (Also, modern 'tar' seems to figure out compression on its own, which is nice if you don't know ahead of time if your input will be gz or bz2 or xz.)
-n