Literally everything in canon up until that point suggests that she shouldn't have been able to do the things she was doing. That's why it stands out as a glaring problem for so many. To myself and many others, Rey learned how to sprint before she could walk.
Ren committed to the Dark Side by killing Han. If there was a moment of weakness, it was when Chewie shot him - which was something he should have been able to defend himself against. After that, he wasted no time in hunting down Rey and Finn and confronting them. The fanciful choreography he maintains even then suggests that he wasn't compromised mentally. He was still capable of fighting, and did so deliberately with Finn, and with enough patience to hold back against Rey to recruit her. I am sure Ren will be processing killing Han in the sequels, but he had it bottled away at that point.
Vader, Palpatine, Dooku and Maul are all powerful Dark Side users and they didn't need to stay angry. They just needed to channel their hate. Ren was angry when he confronted Finn and Rey, he didn't need to make himself any angrier. He was already pissed off.