[vba] Quickest way to clear all sheet contents VBA

The .Cells range isn't limited to ones that are being used, so your code is clearing the content of 1,048,576 rows and 16,384 columns - 17,179,869,184 total cells. That's going to take a while. Just clear the UsedRange instead:

Sheets("Zeros").UsedRange.ClearContents

Alternately, you can delete the sheet and re-add it:

Application.DisplayAlerts = False
Sheets("Zeros").Delete
Application.DisplayAlerts = True
Dim sheet As Worksheet
Set sheet = Sheets.Add
sheet.Name = "Zeros"