I recently downloaded a free version of CintaNotes, just because it appeals to me on so many levels. When I activated Simplenote synchronization, almost 11,000 notes appeared in it. And I can testify that it has absolutely no problem dealing with that many notes.
The notes must have come from previous nvAlt adventures with my ConnectedText exports. I thought I had deleted them, but I must not have. So they did show up in CintaNotes. However, the joy did not last long, as I managed to hose the database by trying to do massive changes to entries, using a keyboard macro program.
In any case, I would suggest that the author of the program look more deeply into SimpleNote synchronization for getting stuff into CintaNotes. The only reason someone like me cannot use it as a serious alternative is that I have many electronic notes that I cannot import into the program's database. (Nor will it be possioble to export them in a format usable in another program.)
1. Export also may be a major problem with TiddlyWiki, but I was still not able to test it.