Hi @lhartmann, welcome to AppImage.
Please use appimagetool rather than AppImageAssistant, the result will be squashfs-based rather than xorriso ISO9660 zisofs AppImages. mksquashfs does support multiple cores for compression. I could imagine that this alone will speed up compression times considerably.
For testing though, an AppImage should not behave noticeably differently from an AppDir when you call AppRun from outside of the AppDir (e.g., current working directory is outside of the AppDir), and the AppDir is read-only (e.g., on a disk mounted read-only).