When a person studies topics related to violence or disaster such as the laws of mourning, he should not dwell much in these places. He should not ponder and think too deeply into these sections of Torah, since thoughts of the mind are very powerful and influential. Therefore when a person reaches these sections of Torah, he should study and pass through them as quickly as possible.
[Editors note: Once we have seen that there are parts of Torah that can be studied very quickly, the same probably applies to the Laws of Ishus, Tznius, Nidah etc. for people that have not yet purified their minds and can not control their thoughts. If any one can source this in the teachings of Rabbanue, please add it to the comments of this post.]
(Sichos Haran 8)