Are they sore or are you getting hot spots?
I use Shimano M520 pedals on my road bike, it is a MTB pedal/SPD cleat, double sided entry. I get hot spots with these pedals on hot days or if I ride a lot a few days in a row.
My hybrid uses 525 or 530 pedals (can't remember). It is also a dual sided SPD cleat pedal, but has an entire platform on it, can be ridden with or without cleats. I do not get hot spots when using these pedals.
I have read it can be shoes (size/fit/insole/sole stiffness), cleat position, foot swelling from heat, sock selection and entire bike fit. I do not know enough about the science/art of bike fitting, others can contribute to that.
As far as making the bike feel "lighter" do you mean you feel you are working too hard for a given result? If so, improper position can cause an inefficient use of muscles, as I understand it.