Quoted from this post. 
GRUB is early bootstage. On a device with an encrypted /boot there is no OS (no Linux kernel) available yet. Unfortunately Grub’s implementation is really slow on most machines, since (unlike the kernel) it can only do pure software decryption or AES-NI, not SSE-accelerated decryption. Thus your first grub unlock stage will probably take multiple times longer than your set iter-time; multiple in this context meaning it could take 10 or more times longer than the calculated default 2 seconds. So the encryption will probably add about 20sec to the boot time for most users.