[pcwalton]

Look at Cranelift [1], in particular "Cranelift compared to LLVM" [2]. Cranelift is in some ways an effort to rearchitect LLVM for faster compilation speed, written by some longtime LLVM contributors. For example, Cranelift has only one IR, which lets it avoid rebuilding trees over and over, and the IR is stored tightly packed instead of scattered throughout memory.

[1]: https://github.com/CraneStation/cranelift

[2]: https://cranelift.readthedocs.io/en/latest/compare-llvm.html

[mshockwave]

I think the one of the key differences is that Cranelift are more low-level