excel spreadsheets format directly from spreadsheetml, it's one of the formats it will save and load straight from, there is no conversion. I have tried big files with no issues and I can make sheets exactly as I want them with the xml method. I did give html a go but although excel will open it the way it will layout is left up to the gods.

also some of the reports I do include conditional formatting and other features of excel which means xml is my only route.

if you want to know how long it will take to load a big spreadsheet try saving one from excel in xml format (it's one of the saveas options) then open it again.