Very bad performance for update from synchronize view

We are experiencing very bad performance when performing an update
triggered from the synchronize view. When performed from the Java
perspective the update takes 23 s, when performed from the synchronize
view it takes > 1300s (I don't know how much longer since I went to
lunch and when I came back an hour later it was finished).

We are using 0.9.34. I tried JavaHL and JavaSVN, but it doesn't seem to
make a difference (note: it's not about the synchronize procedure itself
but about an update triggered from synchronize).

The strange thing is that I can't spot a bottleneck: the client CPU is
idle, the network connection is idle, the server CPU is idle, and so is
the server disk. Everything seems to wait on something but I don't know
what. Even stranger, the operation is faster when performed on a DSL
line than when performed on a 100MBit/s network.

Has anyone seen a similar phenomenon? Any ideas what the reason could be?

-- 
Bernd Rinn