Speed of dd depends on skip/seek values?

For backup, I'm experimenting with dd to copy my internal HD to an
external USB HD. I get the feeling that not only bs, but also skip and
seek parameters greatly determine the overall speed.

Examples, with "if=/dev/sda of=/dev/sdb" omitted for brevity:
- 750 GB: dd bs=8192 count=91571823 takes 07h12m
- 535 GB: dd bs=1440256 count=371567 skip=149282 seek=43831 takes
07h08m

In the first command (seek=skip=0), I may increase bs to 64k or 1M or
2M without significant impact on execution time. Therefore, I think
the slow speed of the second command cannot be caused by too large a
bs.

Is the speed decrease caused by the fact that blocks are not aligned
on 8k or 64k boundaries? Or is something else causing this?

Rotaluclac