Ashley I can confirm that I has many issues with large event sheets when I imported my C2 project, I had to split some of them because it was unbearably slow.
I haven't opened it in like 3 months, so I decided to give it a go and see something changed.
The good news is it seems to work faster, though still a bit slow with collapsing events with many sub-events.
But I did encountered a few bugs on the way.
1. When I tried to open the project I got a message that I'm missing the greenworks plugin, which I didn't remove. Don't know what happened to it, but I reinstalled it and that's that.
2. When I tried opening my project (local folder project) it got stuck on the loading screen (no percentage counting showed at any point, just the text "opening") and it never stops.
I tried to switch to the stable branch (I'm working with the NWjs version, 0.46.3 with the new package.json), but there's no open local folder in the stable branch, only open local file... ?
So I packed it into a zip and renamed it, and it opened just fine on both the stable and beta branches.
I tried re-saving the project in a new directory and open it up, still not working...
Also when I try to open a .c3proj file straight from windows, I get a message that C3 can't open that file and can only open single local files, but I'm pretty sure it worked before.