Any application that gets loaded down with a large number of tasks to might become non-responsive until if finishes processing those tasks. This is not unique to FileMaker.
I think your are likely correct that you have too many unstored calculations on your layout for the environment in which you are using. (This is a bigger problem for WAN or iOS environments than FileMaker clients.)
But also check to see if you have any scripts running at this time, particularly any scripts that use Refresh Window [Flush Cached Join Results]
To see what parts of your layout might be most responsible for the delays, try making a copy of your layout where you can selectively remove different parts of your layout objects and see which have the greatest impact on your system responsiveness. That will help you focus on the parts of your system design that most need improvement.
Note that conditional format settings are also essentially the same thing as an unstored calculation field in this situation so check them as well.
The most likely fields that may slow down your screen updates:
Unstored calculations that use aggregate functions such as Sum, Count, Average, etc. That compute a value from large numbers of related records.
Summary fields from related tables that compute values from large numbers of records
Summary fields from the layout's table when your found set is very large.
Thanks for the clues... I will let you know if I find what was the "problem"
Filtered portals can also be a source of the "slows..."