Pro Tip: never refer to an entire column in an array formula
If you want to bring a good spreadsheet to a screaming halt, use an array formula that points to an entire column. Even better, copy that formula down a column or two and across a couple rows. Once you’re done with that madness, copy the sheet 11 times and adjust the names to match months.
Arrays formulas are one of the fastest ways to have Excel crunch through data. Their only pitfall is that they choose to process the entire range they are given. They are not optimized in the same way that
SUM is when you point to a whole column. With modern versions of Excel, this means that an array formula must process 1 million+ cells for every column you refer to.
The solution: find some way to decrease the size of the range. Several choices: