There are 2 factors here:
1) I was being conservative on my promises. For instance, we have users who reported GHA runtimes going down from ~25m to ~9m [1] 2) Local builds have the amazing advantage of being able to cache subsequent runs. CI workflows are ephemeral. This introduces a performance penalty. However, we are working on something that automagically caches some builds, especially container builds, that would enable huge benefits of 2x-10x further.
Hope this clarifies!