Can we reduce bit by bit cloning time from source HDD to destination SSD by unallocating some memory from source HDD?

Partitions represent completely independent disk areas. If you perform a block-level clone of the C: partition, your cloning tool will only need to care about the 465 GB that was assigned to the C: partition, and won't even look at the rest.

(If you perform a block-level clone of the whole disk, it'll still spend 90% of the time cloning the C: partition, and then it'll quickly run out of space because your new SSD is smaller than the old HDD.)

What will make the process faster is either file-level cloning (e.g. WIM imaging) or filesystem-aware block cloning. For example, the Linux ntfsclone tool (which is used by CloneZilla) performs a bit-by-bit clone but it'll skip all blocks which have no files or other NTFS data stored on them. I would assume that commercial Windows tools do the same.