Speed Up My Worksheet. The Entire Thing Relies On Now()

trying to speed up my workbook, because all of the formulas depend on time and the now() function. It takes forever to add/change anything.

Here is a general description of my worksheet. It's for a game called tribal wars (online browser mmo). You have towns around you, which create resources to plunder. This resources build up over time, thus I need to use the now() function in basically every formula. Each row finds the distance to that location, the time since last attack, the estimated resources built up since the attack, and the number of troops to send.

If only there was some way to update now() only every 1 to 5 min instead of every time I hit a key...