slow upload with nfs over ssh tunnel

Hi,
I have a problem with a terrible slow upload to the nfs server when the
nfs protocol is passed through a ssh tunnel.
In detail: I have two machines in LAN connected via ssh tunnel

client:> ssh -c blowfish -L 7777:server:2049 -L 7778:server:887
server

and a nfs mount

client:>mount -t nfs -o
tcp,port=7777,mountport=7778,rsize=8192,wsize=8192,intr,noacl
localhost:/share /mnt

downloading from server is quite fast

client:>dd if=/mnt/bigfile of=/dev/null
..... 102400000 bytes (102 MB) copied, 2.60933 seconds, 39.2 MB/s

but upload to server is terribly slow

client:>dd if=/dev/zero of=/mnt/zerofile bs=1024 count=100000
...... 102400000 bytes (102 MB) copied, 72.0704 seconds, 1.4 MB/s

However, simple netcat through a ssh tunnel has the equal (high) speed
in both directions, as well as scp or plain nfs connection.
Both machines run Suse Linux 9.3 with kernel 2.6.18

Any hint is welcome.
Thanks,
C.