My previous phone was a non-smartphone (SonyEricsson C901) and could handle a total of a few thousand messages (maybe 3-4k) so I expected a smart phone to be able to handle several times more with more memory/CPU.

To the defense of N9 it seems to only be a problem within the same conversation as the others seemed to work flawless. I deleted the specific conversation today and it is a lot more responsive now. Yesterday I had some problems with loosing connection to the mobile network and they also seem to be gone now, don't know if they are related somehow.

The only problem I see with many messages is the indexing, the overall responsiveness of the browsing is not that bad. I do however miss a way to scroll better, it takes quite some time to scroll to the top A way to disable the smartsearch/indexing would be great as I never use it.

My friends using Android (2.3.x) appears to have even more serious problems with the default SMS applications. Reaching about 10k messages it starts to crach, requiring reboot, messages disappearing, etc. But on Android there are alternative applications which can handle large amounts better. Atleast this is my experience after talking to some other heavy SMS users.