Hello Hans-Juergen,
here are some details about my use case, which is similar to yours.
I'm using BaseX to insert the live public Twitter Stream into databases (see Wiki Entry [1]).
One Twitter message is around 4 kb of size and i'm able to insert about 2000 of them per second
using single XQuery Update inserts. So that would probably be working out for you, too.
If you
use bulk inserts, like caching the items in a item list and running one XQuery Update for all of them, the amount of inserts would also increase.
thus made available for querying
this could be a bigger problem, cause as long as you are writing items into the database (which will never stop in your use case), the readers are blocked.
And if one of your readers will be running, the writers are blocked.
Hope this helps,
Andreas