Hi Nathan.
What is the reference point for answering "What's changed", from when?
I'm running 7.3 and compared to 7.2.1 there's a lot that's been fixed. Only a few issues with it which you can track in GitHub.
Holes? no, not so far as I can tell.
No idea, I run a cloud security company and don't read any reports of Dolphin hacks.
No idea, as above.
Speed of the site is relative to how you implement faster Web hosting.
The flash chat, aka flash anything, I disabled immediately upon install. Boonex have started an integration with Chat+ (Rocket.Chat) which works but can be enhanced a bit more with branding.