What is "target decision date" in sciencedirect-publishing journal?
Unfortunately it is all too common for some reviewers to be delinquent in their manuscript returns. In particular in traditional academic publishers. Often reviewers are professors/researchers with multiple hats in academia including writing up their own research.  I would agree with the previous answer that a target decision date is a way for editors to try and push all reviewers to respond. 

One example where the turnover is quick(er) is the highly structure online journals like Frontier or PLOS. In the former the turnover is ±14 days for most journals and article submission. The second review process is also around 14 days so all in all about 5-6 week turnaround depending on the quality of the manuscript.