Iterating worksheets and cells, copying data to summary sheet
I have found the answer. What seems to have cause the issue is these lines of code:

Set copyRng = xSheet.Range("A:A")
Set copyRng = xSheet.Cells


All I wanted was to access the entire sheet with all of its non-empty cells. This seems to have been the wrong way of doing that since the code does in fact only take mere fractions of a second if I give it a clear defined range such as "A1:A100" in case of the first instance.