Excel Workbooks with external workbook links loading extremely slowly

I fixed this issue for myself by changing from each cell having an individual formula, to the sheet using an array instead. 

You do this by selecting all of the cells you want to take from the other document, going to the other document, selecting all of the data on that side, then hitting Ctrl + Shift + Enter.

The formula will end up looking like this

{=path/[data_document.xlsx]Sheet1'!A1:Z500}

FAIR WARNING: If you do this over empty cells, they will become 0 instead of empty, so if you have code that expects a string, you will get a data type mismatch error.