@pnotova you will have to make sure all the loops are correctly wired. You can select one loop’s current focus with right click. Sometimes it can make sense to put a Cache node in fron of the loop. Also there have been cases where it was best to encapsulate the inner content of a loop in a component in order to make sure what to execute when if you have nested loops.
On performance and KNIME in general I can offer this article: