If reading documentation and looking into implementation details doesn't help, I welcome you to measure ARC overhead and observe the numbers with your own eyes at least once, especially in multi-threaded scenario. Might also read one of the previously posted links still, surely something will work eventually.
I’m not saying ARC is fast. Reference counting is slower than GC if what you want is throughput, unless you somehow manage to do very coarse grained RC, which is what you get in most C++ code (but then it’s far from safe).
The GC<->legacy-C interoperability problem is unavoidable. I’ve been around the block on that problem as someone who writes GCs for a living. It always devolves to some sort of ugliness.
Anyway, the fact that Vala uses GObject RC semantics is a point in favor of Vala. Folks who go that route, as Swift also did, should just be honest with themselves about the tradeoff: simpler interop, worse throughput.
