Its not only Prime 9. PTC had promised to make Prime faster with version 11, but I have not noticed any significant improvement.
In MC15 only the visible part on screen was recalculated and only when you started scrolling down, the calculation continued, if necessary.
This behaviour made Mathcad feel faster reacting (actually it really was faster anyway). Sometimes this behaviour was problematic, especially when opening a large sheet as scrolling was slow because of the continuous recalculation of the sheet. So it often was advised that when opening a large sheet you should fast scroll down to the end of the sheet and wait until Mathcad finishes its calculations. Then moving in the sheet was fast and without disturbance.
In Prime we have no auto-calculation when opening a file, we see the results when the sheet was saved. But changing any variable at the top makes the whole sheet recalculate which can take some time for larger sheets.
Even worse Prime is designed such a way that this recalculation of the sheet is not aborted when the variable or another one is changed while recalc is still ongoing. You have to wait for the recalc to finish and then Prime will start recalc again because the variable has changed from anew.
The behaviour is extremely annoying in large sheets and makes advanced controls like sliders useless in calculation-intensive worksheets because moving the slider would trigger a recalculation multiple times.
In Prime 11 there is new "manual" calculation mode which basically means that auto-calculation is turned off and you can press "Calculate Now", and the help says "This button calculates all math regions that require a recalculation from the first page of the worksheets until the last visible region in the worksheet." . This may help a little bit in some situation but does not cure the main problem, I guess. See About Controlling a Region Calculation
