It's designed to scale up well, and should do better with a few bits of caching enabled (and we plan to expand the cache layer). However, please keep in mind that it hasn't been stressed to extreme levels yet.
Search will always be the big challenge. We do not currently have a Sphinx search system, though we hope that it won't be that hard to add to our search handler at some point.
It takes us currently 3 beefy servers all cached to the hilt to be able to run our forum and that is with Sphinx integration. I dare not try to use the internal search function as it is an almost guaranteed forum killer with the amount of data and traffic we see on a regular basis.
The thing is, it goes well, and goes fast .. It did not take long to get 150k done. But the load simply went from 0.09 to .. way toooo much. Since there currently is not 'pause/continue' button - but you can Exit the rebuilder, so when load goes up you can quit it. I think ideally doing a shell query or script to handle it would perhaps resolve that problem. I am not sure at this point. Maybe my server with xeon cpus and 2gb ram is just not enough.