Please not that using svn_log without giving revisions is much, much slower then with revisions. Examples:
$ time php -r "svn_log('http://localhost/svn/shopadsl');"
real    0m2.140s
user    0m0.140s
sys     0m0.000s
VS
$ time php -r "svn_log('http://localhost/svn/shopadsl', 0, 45);"
real    0m0.063s
user    0m0.024s
sys     0m0.016s