What is the right approach to research when goal is not to publish in top conferences? I am theoretical computer scientist. I am currently working in a local university in south  asian country. I have tried my best in the past to publish the research in good venue but have been failed. Now I am okay or fine with publishing in low tier conferences.
I have one doubt about my approach. I read top conference papers like STOC,FOCS,SODA and understand them if they seems interesting to me. Then I try to rephrase a research mentioned in the paper. After working on that question results in a paper.
The problem with approach is it takes quite a time to read and understand papers from top conferences as they are lengthy and nontrivial. This some time takes 4-5 months and total time for one paper is around 6-7 months. Many times it seems after reading these papers that there is no scope of improvement. In that case to me it appears that I should have read a paper of a conference which is not top tier. Lets say paper X is published in tier 2 conference then in most of the cases the reason it has been published in tier 2 is incomplete solution (partial solution to a problem).
Now the question is How to research if my goal is to publish and not to bother about ranking of the conference in efficient manner (time and energy)
I am guessing, but I think you haven't sufficiently specialized.
Most theoretical computer scientists are not specializing in something as broad as graph algorithms.  They are specializing in something extremely narrow, say specifically in list coloring algorithms, or specifically in adaptations of Lenstra's integer programming algorithm.  Your aim should be to specialize in something so narrow that you are the world expert in that very narrow area.
Don't try to understand anyone else's work completely; you only need to understand it well enough to see how it contributes to your work.  Because you have focused on your area well enough, you should be able to read (the relevant parts of) papers much faster.
The danger in doing this is that you end up producing work that no one else is interested in, but, as long as it remains legitimate research, it should be publishable in lower-tier conferences.