As far as i can understand from my experience of working in a project, I can say that performance tuning relates to increasing the performance of the application by means of optimizing CPU utilization, flexible cache management and such other aspects.
I can give you certain examples like suppose you have a dashboard where you are displaying multiple results by hitting some RD's in various grids. So, if you try to load the dashboard at one go then it might take a pretty high time as we need to fetch results from DB. So, here we can apply defer load check on the layout containing grids so that at once we need not load entire data.
Some pega internal tools like PAL,DB tracer can be effectively used in this regard to find out the queries which take a pretty good time in retrieving results.