ThinLTO can be pretty quick if you have enough cores, it might work. Not sure how well the LTO objects compress against each other when you have small changes to them. It might work reasonably.
manyclangs is optimized to provide you with a binary quickly. The binary is not necessarily itself optimized to be fast, because it's expected that a developer might want to access any version of it for the purposes of testing whether some input manifests a bug or has a particular codegen output. In that scenario, it's likely that the developer is able to reduce the size of the input such that the speed of the compiler itself is not terribly significant in the overall runtime. Therefore, I don't see LTO for manyclangs as such a significant win. But it is still hoped that the overall end-to-end runtime is good, and the binaries are optimized, just not with LTO.
