Part of issue with 15 is the graph displays. It is very slow when I scroll, especially if I have multiple lines on the same graph.
I guess this is only the case when you open the sheet the first time or change a parameter which forces a recalculation. MC15 beahves differently whne autocalc is on than Prime. When you open a Prime sheet you see the results of the last calculation before saving the sheet. MC 15 will automatically recalculate a sheet (when autocalc is set on) when you open it, but it only calculates up to the part of the sheet which is shown. So when you scroll down further, recalculation continues and this sometimes feels slow. A general advice for an extensive and complex sheet is to immedeatly scroll down to the end when you opened it and wait until the complete sheet finished calculating.
Generally it would be hard to believe that MC15 is slower than Prime.