We have spreadsheets where the calculations are done with (custom Excel v4 .xla type) macros. Whenever we run those spreadsheets on later Excel version it (seemingly) takes forever to finish a run.

Last time we used the spreadsheets having the same problem we re-installed v5. We have to make the same calculations again, have the same problem and again re-installed v5. The identical run took 4.2 minutes. (Initial run was 12 hours)

Is there a way to speed this up on newer versions or an easy conversion to VBA without recreating macros?