Everybody cares about compilation speed. What would be interesting is how much time in percentage is spent optimizing it though. Does any one have any hard daya here?

We don't track time of who works on what, so there's no way to answer this specific question with hard data.

They haven't made it a priority until the last year or two, no efforts were made to improve it before then.

This is simply not true.

For example, here's the 2017 roadmap, which was authored in 2016 https://github.com/rust-lang/rfcs/blob/master/text/1774-road...

> The edit-compile-debug cycle in Rust takes too long, and it's one of the complaints we hear most often from production users. We've laid down a good foundation with MIR (now turned on by default) and incremental compilation (which recently hit alpha). But we need to continue pushing hard to actually deliver the improvements. And to fully address the problem, the improvement needs to apply to large Rust projects, not just small or mid-sized benchmarks.

We didn't do these sorts of yearly plans before that, but note that it isn't just talking about what should be done then, but the work that was being done on this previously.