[Feature request] Support .xlsb (Excel Binary Workbook)

[Taiizor]

.xlsx loads 4 times longer than .xlsb and saves 2 times slower and has 1.5 times a bigger file. Reportedly a user from leading forums Stackoverflow tested this on a generated worksheet with 10’000 rows * 1’000 columns = 10’000’000 (10^7) cells of simple chained =…+1 formulas:

	.xlsx	.xlsb
loading time	165s	43s
saving time	115s	61s
file size	91Mb	65Mb

Reference: Click Me

[JanKallman]

We might look at this long term, but it's not for the coming versions as it requires a totally new format for many of the package parts.