Comment by estebank

There are multiple avenues to improve both first and incremental compiles. They "just" require large architectural changes that may yield marginal improvements, so it's hard to get those projects off the ground. But after the last project all-hands I do expect at least one of these to be pursued, of not more.

There are cases where cargo recompiles crates "unnecessarily", cargo+rustc could invert the compilation pyramid to start at the root and then only compile reachable items (similar effect to LTO, but can produce a binary if an item with a compile error isn't evaluated, improving clean compiles), having better communication with linkers and having access to incremental linking would be a boon for incremental compiles, etc.