I don't see any reasons why an ordinary Windows scan should be slower than boot-time scan; on contrary, the boot-time scan will be slower (unless it "makes it up" by supporting less archives, thus scanning "faster").
The boot-time scan is not meant for regular scanning (it is somehow limited in archive support, as well as removal of registry associations, etc.). It should be used when you suspect an infection.
No, the release will certainly not be delayed because of a 64bit boot-time scan. And it's not just a boot-time scan - the whole program will have to be 64bit natively - and it will take quite some development time.