1. I've not seen any reports of this for a long time. I can open 400mb projects on a phone. Please file a bug if you still have issues.
2. Try saving to the local browser instead.
3. The vast majority of keyboard shortcuts in C3 are identical to those in C2 (see Keyboard shortcuts). We've only had to change a few where it performs a browser function like closing the tab, which browser makers don't let pages override.
4. Not seen any cases of this before, please file an issue if you can reproduce them.
5. You can post suggested improvements here: https://construct3.ideas.aha.io
6. Try changing the theme.