Unfortunately this is a non-trivial problem we are working on resolving. It occurs when trying to pack a crystal structure in a highly symmetrical spacegroup that contains large, highly symmetrical molecules on a special position or polymeric molecules.
The speed of packing of a single unit cell can be improved by opening the Packing and Slicing dialogue via Calculate, Packing/Slicing, changing the Include atoms radio button to ...that Fit. Once this setting has been changed, activate the Pack tickbox. Mercury will display packing but only for the atoms whose coordinates are at or within the boundaries of the packing range specified thus speeding up the display of packing.
If you are packing more than one unit cell, we would recommend that you first open Mercury (without loading your structure), open the Packing and Slicing dialogue as above and change all appropriate settings (i.e. the Include atoms setting should read ...that Fit, activate the Pack tick box then change the a, b and c values to those required). You can then read your structure in. Mercury will still take a reasonable amount of time to process the settings and display the packing (also dependent on what your settings for a, b and c are) however Mercury will eventually display the packing.