Any issues with packet updates?

Question summary:  Are there any issues when mangling
packets as they pass thru the bridge, changing TCP/UDP
payload content and size?  The symptoms are very, very
slow thruput (sometimes higher volume bursts) but
traffic eventually completing.

Details
I am using RH 8.0, with a 2.4.20 kernel containing the
bridge 0.0.10 and ebtables 2.0.003 loaded.
I have a pair of machines, both with 2 ether cards. 
Each has their 'intranet' on eth0 and their 'internet'
on eth1 (tho the 2 eth1s are just in a hub connecting
the 2).
I have a userspace (iptables) QUEUE target on my dual
ether machine, loaded in PREROUTING (marked for when
incoming to the physical interface) and POSTROUTING
(marked for when outgoing on the physical interface)
on eth1.  The jist is basically all outgoing packets
(POSTROUTING) between the hosts will be marked on the
outdev_name field in the userspace module and all
incoming packets (PREROUTING) will be marked on the
indev_name field in the userpace module.  

Overall, the intent of the userspace module is to
mangle  TCP/UDP packets on the 'internet'.  Each side
mangles on output, demangles on input, changing both
the tcp/udp payload and it's size.  My configuration,
when the mangling is turned off, works perfectly fine
with great thruput.  As soon as I activate the
mangling, it slows to a crawl, but does get there and
is received correctly.  In router mode, everything
looks like it works perfectly fine.


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