I came to this section of the forum planning to post about my DD about a similar thing and found your post. My DD is 12y old and we have a similar issue with her. She will take forever to complete a task. On occasion, when she has gone to have a shower, it will easily take over an hour. Once, she went in for a shower when I was out. DH left the house when I got back home. An hour later, she was still not out. When I knocked to check/chase up, I realised she had still not gotten into the shower and just started getting ready to go in!!! I later learnt that she had already been in for an hour when my DH left the house. He had assumed she was finishing when he left!!!
We have observed her capable of getting things done; the problem seems to be that she will just not get on and do whatever it is she is meant to be doing!!!
Nevertheless, I think we have more or less cracked getting ready in the morning. It happened 2-3months ago. I decided to slack myself and ask her to keep the time for us to get ready. So, SHE was to tell me when it was 7am, so that I could get downstairs to get ready to leave, and SHE was to remind us to leave the house at 7:10am. I have slacked at times (occasionally intentionally 
). I has meant that she is managing to time-keep better and leave the house without me chasing her with repeated yells and stress. My next plan is to get her an alarm clock so she can get herself out of bed herself without us waking her up. Wish us luck!!!
Any advice about the shower situation is most welcomed.
PS- We've just restricted wifi access and started to take her phone off her when she gets in from school, returning it in the morning when she leaves for school. Hopefully, it will mean she has fewer distractions for getting other things done (one lives in hope).