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Does it end process or ask you to edit? If it ask you to edit there might be an overlap, that has happened to me. On the other hand if it lets you continue it may just be verifying you want that big of a pattern. I hope this helps.
When there are large number of occurances then optimized compute option works best in Pattern. The patterning features (like extrusion on top of extrusion) have simple relationship to the base body.
If you are producing large arrays there are various ways that Inventor can regenerate the features, the default is 'Identical'. When it suggests 'Optimised' open the flyout at the bottom of the array command window & select optimised. This changes the way that Inventor handles the array & allows it to work quicker.