Turn out that the ruby repo is something special in that it has very deep histories of relatively few, very small files combined with one huge changelog file (the latter taking up ~75% of the repo). See below for details.
Also, please note that your exports contained >500000 files. Using 16MB of cache with that project size *may* not be an adequate setup. Upping that to insane 256MB (roughly what 1.6 would use anyway), gives much better numbers. However, there is hardly a difference between f6 and f7 in these runs. Here my measurements with svn: under Linux: null-export svn://localhost/ruby/trunk 461 directories 4,263 files 48,880,945 bytes in files 23,783 properties 324,660 bytes in properties F7 packed 20.249s, all default F6 packed 20.168s, all default F7 packed 14.310s, 256MB cache F6 packed 15.167s, 256MB cache F7 packed 13.649s, 256MB cache -c 0 F6 packed 14.226s, 256MB cache -c 0 F7 packed 13.478s, 256MB cache -c 0 --cache-revprops yes F6 packed 13.223s, 256MB cache -c 0 --cache-revprops yes export svn://localhost/ruby/ 54,641 directories 512,580 files 4,558,565,078 bytes in files 3,077,645 properties 45,809,954 bytes in properties F7 packed 763.520s, all default F6 packed 785.913s, all default F7 packed 153.520s, 256MB cache F6 packed 152.746s, 256MB cache F7 packed 64.695s, 256MB cache -c 0 F6 packed 65.505s, 256MB cache -c 0 F7 packed 55.534s, 256MB cache -c 0 --cache-revprops yes F6 packed 56.697s, 256MB cache -c 0 --cache-revprops yes null-log svn://localhost/ruby/ 46,054 revisions 177,176 msg lines 0 changes F7 packed 58.494s, all defaults F6 packed 58.852s, all defaults F7 packed 2.153s, --cache-revprops yes F6 packed 2.470s, --cache-revprops yes