Drupal includes many files. When DocumentRoot is hosted on NFS, include calls are slow, because too many lstat() calls which are slow on NFS.
Solution is to increase realpath_cache_size in php.ini.
; Determines the size of the realpath cache to be used by PHP. This value should
; be increased on systems where PHP opens many files to reflect the quantity of
; the file operations performed.
; Duration of time, in seconds for which to cache realpath information for a given
; file or directory. For systems with rarely changing files, consider increasing this
There is opened bug (tagged as wontfix), which causes realpath cache is disabled when open_basedir or safe_mode are in use.
They must be disabled to realpath_cache to work.
On tested site page load time was 12 seconds, after change is just 2 seconds.
Detailed analyze can be found in original article.
There is a PHP extension enabling realpath caching on NFS: