A fleeting thought (memory, simple inference etc.) have a light feel.
Opinion deciding thoughts are "heavier" they do take hold on you. Your physiology etc. are more involved.
Plans are much more heavy. You feel you are working. And those are the hardest to "let go" of.
Ps. Ego depletion research has shown that making decisions is depleting. Also, changing attitudes (related to the opinion part above). Interesting.
