Since the very beginning of GHA testing, we have been running JDK tests with release bits. This was -- as far as I remember -- to optimize the testing time with the slow (at the time) GHA infrastructure. This, however, is not great for several reasons:

1. Our minimal integration bar is tier1. GHA helps us to maintain this bar. Anecdotally, most of Hotspot folks I know run all tier1 tests in fastdebug mode, because they hunt for Hotspot-related bugs. So, GHA that runs JDK tests in release is set to miss some of the things that would fail in tier1 fastdebug on developer machines.

2. Some JDK tests verify not only the Java side of things, but also the runtime parts: the compiler support (e.g. intrinsics, high- and low-level optimizations, etc), native code (which has more native asserts), etc. The very prominent examples of this are FFI tests, Virtual Thread tests, etc. We actually want to run JDK tests with fastdebug to get fuller coverage for JDK code.