if your server is CentOS, the clients are Mac and performance is a significant concern then I'd tend to avoid AFP and - as you've discovered - WebDAV. The only significant win with AFP is that making Time Machine work is easier, but everything else ends up harder.
Your two primary (mainstream) options are CIFS/SMB or NFS. The former has the advantage of being more readily compatible with Windows and would require setting up Samba on the CentOS box (not a big deal, also adds printing support, etc). The latter (NFS) probably has slightly fewer moving parts and I've had great luck with it on a lot of OS X boxes. The client configuration is a bit more obscure, but no biggie once you've figured it out once.
One additional thing to consider is how you're maintaining identity information. For NFS you'll need some external mechanism for synchronizing user ID's (i.e. LDAP) and such. Samba is a lot more self-contained - can use AD, LDAP, built-in username/password, etc. If you already have some sort of centralized user management / login setup on your CentOS box then NFS is probably the best option. Otherwise the best option is a function of what you have the most comfort supporting, but overall support for Samba is going to be a lot more prevalent.
Performance between NFS and CIFS/SMB is basically a wash when both have been appropriately configured.