A missing build will only slow testing migration if the package wasn't built when the unstable testing delay is over. Since almost all uploads to NEW are low urgency, the build would need to take over 10 days to affect the time to testing migration. To speed up testing migration due to missing builds, it would be much more efficient to work on buildd redundancy (or other improvements to the buildd network).That probably won't make much time difference on "fast" archs (i386, amd64 etc), but on slower ones like mips, mipsel etc (those sometimes hold up testing transition :().