Just had a look, and it turns out that's including running LLVM's optimizer as well, which takes roughly half the time. It's basically doing the equivalent of -O2 currently. That's probably more aggressive than what's needed, but I haven't taken the time to fine-tune it yet.

Other than that, you're right that it might be a bit high. There's very likely parts of the lowering code which can be optimized, I just haven't had the need to yet.