My saves, exports and pretty much everything takes longer when I have a mountain of takes and the grows massively with intricate comps in play. I can really say whether Flattening helps at all but I haven't noticed any significant improvement after doing so for most stuff (might be quicker for saves and exports).
The #1 thing that improves the speed of EVERYTHING in my project is copying my final, flattened take to a cloned track and archiving the take laden original version.
If I do this across the entire project I go from a project that...
Takes a second or more to begin playback to a project that starts playback immediately
Makes what I THINK is what is referred to as "zipper noise" when jumping the Now Time to another location during playback into one where no noise occurs when doing so
Exoprts slowly (sometimes more slowly than real time bounce when Fast Bounce is selected) to one that exports quickly
Saves slowly to one that saves quickly
Is glitchy to one that is less glitchy
etc...
This is why I posted a Feature Request recently for us to be able to bulk archive Take Lanes. So you'd Flatten your final or multiple finals and archive all the other take lanes to remove them from processing. This avoids having to do my little "Clone/Archive dance every time I finish a track". Go vote for it.
I did however forget to include in that FR something I've been asking for since X2 which is a "Hide Take Lane" feature so I don't have to look at those piles of takes when I've got my finals but don't lose them or have to clone, archive and hide them the old fashion way.
Meh. Would make things much easier for me and I seriously think this is the type of stuff that might improve stability for take maniacs like myself... thus less complaints on the forum.
Just push it as part of the standard/recommended workflow and I'd bet people would be a lot happier with Sonar. Seriously the improvement is like night and day when I do this stuff... but it's a time consuming PITA especially on large projects.
Cheers.