VBA script to add formulas to Excel is running slow
When working with Excel and writing to cells the most common performance issue come from the sheet being constantly updated in the background while the script is running. In order to gain more speed you should save the state of a few properties for as long as the script is running and restore their values when it finishes.

For example:

Application.ScreenUpdating = False   
Application.DisplayStatusBar = False    
Application.Calculation = xlCalculationManual    
Application.EnableEvents = False


Reference: Excel VBA Performance Coding Best Practices