Dual wifi connections in Bookworm

I've now got two wifi connections on my Pi5 running Bookworm.
Following the latest update a day or two ago both come up
automatically on reboot connecting to the same access point
with the same frequency and channel. Using ifconfig it's
possible to turn them on or off individually.

The internal wifi shows a ping time of 5-10 ms, the USB
external Archer T3U is generally under 2 ms, perhaps because
it reports a stronger (~95% vs 75%) signal strength.

Using both interfaces together seems to result in much worse
performance than either one alone, which I didn't expect at all.
Ping times reach tens of seconds under load, for example. The
effect relliably appears a few seconds after applying the load
using the chromium browser. 

I'm a little surprised that two interfaces are worse than one.
 
Has anybody else seen this behavior?

Thanks for reading,

bob prohaska