paper acceptance probability in case of long peer review time for the 1st decision
It is difficult to say so. Usually it means that the journal is having many papers to handle, referees are not responding in a timely manner or cannot find referees that are familiar with the subject of paper. Of course, I am putting aside the possibility that the journal may have a bad review process as it is usually uncommon for well-known and prestigious journals. 

As a matter of fact, I had a paper that I had submitted with a prestigious journal. It took one year to have a first decision and the decision was rejection. In this case, the journal editor told me that the review process took a long time because they had difficulty with finding referees that were familiar with the particular subject of my paper. 

This question is also somehow related to yours: Editor rejected my paper stating that reviewers had refused to review it long after submission...?