The optimisation passes are expensive (not the largest source of compile time duration though).

Debug mode is designed to build as-fast-as-possible while still being correct, so that you can run your binary (with debug symbols) ASAP.

Overflow checks are present even in release mode, and some write-ups seem to indicate they have less overhead than you’d think.

Rust lets your configure your cargo configs to apply some optimisation passes even in debug, if you wish. There’s also a config to have your dependencies optimised (even in debug) if you want. The Bevy tutorial walks through doing this, as a concrete example.

Yes, optimization is disabled by default in debug mode, which makes your code more debuggable. Overflow checks are also present in debug mode, but removed in release mode. Bounds checking is present in release mode as well as debug mode, but can sometimes be optimized away.

There's also some debug information that is present in the file in debug mode, which leads to a larger binary size, but shouldn't meaningfully affect performance except in very simple/short programs.