When should the xlsm or xlsb formats be used?

Question

Since Excel 2007, Microsoft has split the classical .xls format to several formats (in particular, .xlsx, .xlsm, .xlsb).

Answer 1

.xlsx loads 4 times longer than .xlsb and saves 2 times longer and has 1.5 times a bigger file. I 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    ║  91 MB │  65 MB │
╰──────────────╨────────┴────────╯

_{(Hardware: Core2Duo 2.3 GHz, 4 GB RAM, 5.400 rpm SATA II HD; Windows 7, under somewhat heavy load from other processes.)}

Beside this, there should be no differences. More precisely,

both formats support exactly the same feature set

cites this blog post from 2006-08-29. So maybe the info that .xlsb does not support Ribbon code is newer than the upper citation, but I figure that forum source of yours is just wrong. When cracking open the binary file, it seems to condensedly mimic the OOXML file structure 1-to-1: Blog article from 2006-08-07