Well, that's a million+ cells that the formula has to work on in each column, and then build up an intermediate array of values which will also be a million+, so it will be very sluggish with such long ranges, and if you only have, say, 200k rows of data (which is still a big spreadsheet), then you are getting Excel to do 5 times 4 times more calculations than it needs to. I would suggest using another column to derive the month, and then you can use COUNTIFS - this function is intelligent enough not to calculate beyond the used range, so it will be much faster and you can use full-column references without concern for the speed.

Hope this helps.

Pete