As everyone here, I too consider myself as a slow thinker. Some of the reasons for the same, I believe, are
(i) Always trying to get the bigger/entire picture of the problem
(ii) Trying to increase the precision of the answer for the problem under question
(iii) awareness that there will always be unknown unknowns
(iv) trying to cater to exact needs of others
(iv) Not giving importance to memory
(v) a wandering mind, which wants to catch hold of too many things at the same time.