Happens to Windows and Mac OS too. Every time I boot Dropbox thrashes my disk for 10 minutes while the system is almost completely unresponsive.
In its heyday, Solaris was outstanding in terms of being responsive while simultaneously doing large amounts of I/O. (Or at least that's my perhaps clouded recollection, I haven't used Solaris in over 5 years).
I seriously doubt that any usermode program could overwhelm the OS scheduler like that. What are your use case parameters?
The OS IO scheduler is known to be shitty. That's not an exegeration. You can start swapping because the scheduler does not free unused caches fast enough.
I assume since my data was synced and didn't need to be indexed all over again - I got some savings there. Maybe your dropbox configuration data is corrupted and thats why it needs to index it all again.