smbmount / smbclient : strangely varying transfer speeds

[Achim]

Hello List,
I've experienced the following with both a kubuntu and a FBSD7 client and  
FBSD7 as server:
When i try to copy a file off a *mounted* CIFS/SMB-share I get transfer rates 
below 1 MByte/sec. If i start a second, concurrent transfer i am getting 
transfer rates around 8MB/s on *each* transfer (Gigabit link). As soon as one 
transfer stops, the other is dropping to the old rate below 1MB/s.
Copying TO the server works fine, atleast from the kubuntu client, the fbsd 
client isnt here atm. It speeds up the initial transfer to about 3.5MB/s, 
about half of what a concurrent download does.

A single transfer via smbclient yields ~8MB/s. 

In other Words: 
Performance with a single client is degraded when the client is smbmount and 
downloading. 
With a second transfer in any direction, performance becomes better, to about 
3.5 resp. 8 MB/s depending on the second connection up- or downloading.
Unlike smbmount, single smbclient transfers yield acceptable results.

Anyone with an idea as what to try? 

My wireshark skills aren't too advanced and i could not find any notable 
difference between the captures of each transfer type (single mounted, 
multiple mounted and single smblient), anything i should watch out for?

The machines are connected over a simple soho gigabit switch, no fancy network 
between them.

thanks in advance,
Achim