forum scalability with forum_access

The question is not so much about forum_access but rather about the node access system, which is a Drupal core feature. Every query involving nodes (any nodes!) will be considerably slower if you have any node access module installed, but I can't give you any numbers. I believe the number of nodes is not really the determining factor (except that sites with many nodes tend to have a lot of hits and performance becomes even more important). FA works mostly with roles, and I would guess that having more roles would cause more slow-down than having more nodes.

No followup, assuming the answer contained the required information.

Do you know any "big" forum that uses FA?

I have bad experience with node access (to be exact, the workflow access module), I can say that it is the bottleneck. I had a D6 site with 15K nodes, a few roles (5 or 6) and a hundred row node access table. The site was super slow, load was over 20, it was down to 1 only after I disabled the node access module and reset the node access table.

The node access system was supposed to be retouched in D7, but I didn't see that. That's why I ask for "real experience".