There is also an interesting presentation about how Azul implemented their JIT called Falcon, that is fully based on LLVM. https://www.youtube.com/watch?v=Uqch1rjPls8
Generally LLVM is a nice idea that allows vendors to reuse major components.
Absolutely correct that LLVM is a fair amount slower than C1 or C2. Azul augmented our JITs with a compilation recording & replay mechanism to combat the general start-up problems posed by JITs (including Falcon).
For those who are curious, here's my presentation on the topic: https://2018.jpoint.ru/en/talks/63npsyjpokmukqsag80oia/.