decrypt or no decrypt? topic

Hi I was just wondering if its safe to decrypt files on my nexus six I just saw a video and it says it will speed up my phone and supposedly improve battery life anybody done this yet? And is there a big difference? Is it necessary? And also I want to add a custom recovery and I think I'm gonna use twrp manager but it says unable to write to SD and asks me if I want to partition it should I partition or not? Any help would be awesome and thanks in advance