Oh gosh, where to start:)

- on any calculations that are iterative, - the recacluation rate is limited to something like once per second, - sheets containing over 2000 market positions getting real time quotes grind to a halt. - any time you have more than about 20,000 calls out to a COM component per sheet - large vba functions, it seems vba is about an order of magnitude slower than the corresponding Excel built in methods.

TL/DR large sheets, dde, COM, heavy use of iterative solvers, large amounts of vba, all make excel really slow down.

Having said all that Excel is really awesome, it's just not for big data:)

Suppose somebody made a spreadsheet for big data. What might you do with it?

I believe excel only updates once a second (or whatever the refresh rate is, I remember it being too slow for high frequency trading).