I don’t personally run Loki on a single instance, but in general if you are trying to optimize for one single Loki instance, then you should want to make sure that you write as few chunks to the storage as you reasonably can (you obviously don’t want to keep chunk in memory for too long either), and you want to make sure to split your query as little as possible as well, this usually gives you the biggest gain.
Please see my comments on another thread here: Loki full CPU for a long time and need restart to work again - #6 by tonyswumac. It has some general recommendations there.