You know what the odd thing is? I've used computer for, like, 100 years now (well maybe not quite), but if anything the software, like spreadsheets, are getting worse, not better.
I loaded a few thousand lines of temperature data into a spreadsheet program and it just froze for a minute or so. Things should work better than this in 2013. You could do better than that in 1980.
All I can suggest is learn Lua. With that you can process text files very quickly. You should be able to handle your data, average it, total it, whatever, if half a minute.
(This is for the PC/Mac/Linux, it's not for the Arduino).