Proc macros themselves are slow too. If you compile them in debug mode, they run slowly. If you compile them in release mode, they run faster but take longer to compile. This is especially noticeable with behemoth macros like serde that use the complicated syn parser.

Compiling them in release mode does have an advantage if the proc macro is used a lot in your dep tree, since the faster invocations compensate for the increased compile time. Another option is shipping pre-compiled macros like the serde maintainer tried to do at one point, but there was sufficient (justified) backlash to shipping blobs in that case that it will probably never take off.

Here's a comparison of using serde proc macros for (De)Serialize impls vs pre-generated impls: https://github.com/Arnavion/k8s-openapi/issues/4#issuecommen... In other words the amount of code that is compiled in the end is the same; the time difference is entirely because of proc macro invocation. 5m -> 3m for debug builds, 3.5m -> 3m for release builds. It's from 2018, but the situation is largely unchanged as of today.