Concurrent AFP and SMB traffic: Slow?

Hi all,
I am trying hard to explain an issue I see on the Drobo FS to Drobo support. So I am wondering if anyone else can confirm it. My scenario is as follows:
[]I start a large file download from the Drobo to a Windows PC, using SMB. I get around 30-50 MB/s.
]Then I access the Drobo from the Mac, using AFP (e.g. I start a TimeMachine backup, or I list a directory)
[*]SMB speed drops quickly to ca. 10MB/s on the PC. The transfer even gets interrupted from time to time (which is obvious if I watch a movie on the PC, instead of doing a file transfer).
When I access the Drobo from the Mac using SMB instead of AFP, I don’t see this issue. So I guess it’s not simple disk I/O starvation, but something else (such as AFP using up too much CPU on the Drobo).
Can anyone confirm this behaviour? I am using the newest (1.2.1) firmware, by the way. The last firmware (1.2.0) showed the same behaviour on the PC side, plus AFP was very slow (on the newest firmeware AFP is fast enough).
Thanks! MArtin

I’m leaning toward it still being simple I/O contention, just that samba is handling it better than netatalkd. But I can’t say I have anything to back that up other than a hunch. Neither samba nor netatalkd use any appreciable CPU, so that shouldn’t be a factor.