Drupal files on NFS, performance degredation
Assuming APC is installed; inside php.ini

; http://www.php.net/manual/en/ini.core.php#ini.sect.performance
realpath_cache_size=1M ; 1MB
realpath_cache_ttl=3600 ; 1 hour

; APC
; http://www.php.net/manual/en/apc.configuration.php
apc.ttl=2419200 ; 4 weeks
apc.localcache = 1
apc.localcache.size = 1024
apc.num_files_hint = 2048
apc.user_entries_hint = 128

apc.file_update_protection = 10
apc.stat = 0
apc.stat_ctime = 0
apc.file_md5 = 1

; OPcache
; http://www.php.net/manual/en/opcache.configuration.php
opcache.enable_file_override = 1
opcache.validate_timestamps = 0


Then when your code changes you need to restart Apache or run 

apc_clear_cache('opcode');
opcache_reset();


And for Drupal install the following modules as these are designed to help with the drupal files dir when used on a NFS filesystem
https://drupal.org/project/imageinfo_cache
https://drupal.org/project/advagg
https://drupal.org/project/httprl (will speed up the above 2 modules)