The Excel Team Blog has this to say about the curious .XLSB file format.
Even though we've done a lot of work to make sure that our XML formats open quickly and efficiently, this binary format is still more efficient for Excel to open and save, and can lead to some performance improvements for workbooks that contain a lot of data, or that would require a lot of XML parsing during the Open process.
(In fact, we've found that the new binary format is faster than the old XLS format in many cases.)
Also, there is no macro-free version of this file format – all XLSB files can contain macros (VBA and XLM).
Runtime performance - once loaded into memory, the file format has no effect on application/calculation speed