The printer is an Epson 4900. Now please explain how the printer is associated with manipulation of the Print Module's Custom Package.
I have reset the Lightroom preferences and the problem persists. One additional note: It does not matter whether or not an image is in the cell I am trying to modify. In both cases dragging a cell or changing its size via its handles causes delays. Initially the cell begins to move but soon stops even though the pointer is still in motion. Then it takes 5-10 seconds before any operation can be performed on the cell.
I wish I could tell you which version of either the OS or Lightroom first revealed the problem but I cannot. I am a long-term user of Lightroom and this is a relatively new problem. It just got really annoying today, causing me to ask if anyone else has noticed this effect.
I have had this same problem now for some time on a iMac (Retina 5K, 27-inch, Late 2014) with AMD Radeon R9 M295X 4096 MB. Even when cropping in the develop module I get a similar slow and or jumpy experience. This has been experienced across at least 4 updates of LR and even the latest CC version put out today.