Poor NFS-Performance with > 7.3

Hi List, think i got a strange problem and don´t know what to do. Scenario: nfs-server with suse 7.3, kernel 2.4.16-4GB. clients #1: all suses older or same as 7.3 - no problems with nfs-performance clients #2: all suses newer than 7.3 - VERY poor nfs-performance. i tried a lot mount options to get a better performance, but nothing really did it. Only -o noacl resulted in a measurable improvement. Tried also async, but no effect. even mounting with samba is faster ( but not as fast as with 7.3 ) anyone an idea ? greetings andy