Should I invest more time on old problems or new problems? I am PhD works in theory of computational maths. I am currently after my mid stage. I have written and submitted two papers ( one paper per year) and currently working on one and these days I am full of ideas, but the problem is to publish a high quality research. Please note that I don't work in a team due lack of people which I am the only who is working most of the time. Here are few questions 


If the submitted paper gets rejected, should I work on them more or should just publish them in some low tier journal? Now I am more mature to picking new research problem which seems to me more significant then previous one, so to me it appears that I should invest more time in new problems
If I publish ( low tier) the submitted paper with one more than I can claim my PhD, but that work seems to me less significant. So I will get my PhD early but less quality research. I am not even sure with a longer PhD I will be surely able to publish in top conferences.


The one more problem is I have now problems as well as ideas but energy is low and also workforce is not there. What to do?
My Bayesian advice is to go with the lower prestige journals and move on. Quantity has a quality of its own.  Plus you learn more as you go on.  Don't get snarled up in reviewer, editor debates.  Keep publishing, keep producing.  The quality will come.
BUT also, I would work on your clarity in written communication (using this question as evidence).  That often holds back a borderline paper or allows one to move through.