What to do if a peer-reviewed journal took so much time to review your paper? This happened many times, and following are some cases:


You submit an article to a good journal with high impact factor. Then after 6 months or sometimes more you dont receive their comments. We tried once to contact them to see the updates they replied ( it is currently under review we will reply once finished).
Many times they reply after a long period with (out of scope). 


What is the best action to do in such cases?
Generally Editor takes decision on a Manuscript based on minimum two reviewers comments. So one possible reason is the required reviews are not completed yet. Followings are few points which are responsible for it. 


Editor has contacted many suitable and potential reviewers, but at least two of them has not agreed to act as a reviewer of your manuscript. 
Minimum two reviewers have agreed, but one or both of them are delaying to submit their comments
Sometimes, editor does not satisfy with the one or both reviewers comments, so again he/she send your manuscript to another reviewers.


So there are many things are in behind. But most of the journals generally takes 2-4 months for the first decision and some journal takes more than 6 months and some even more than 1 year depending on above situations..  So if you are in hurry to get this publication and can't wait more, then you may request to the Editor for a final decision at the earliest possible, otherwise you are going to withdraw your manuscript. Since there are number of suitable journal available, so you will definitely find another one.