JPEG compression is really very well understood after all these years, and very efficient, so the few CPU cycles it takes are as nothing compared to the dawdle of the buffer-to-CF writing speed.
Did you pay attention to footnote 3 of the table you referred to? Maximum number of exposures that can be stored in memory buffer. Drops if Optimal quality is selected for JPEG compression, ISO sensitivity is set to H 0.3 or higher, High ISO NR is on when auto ISO sensitivity control is on or ISO sensitivity is set to 2000 or higher, or long exposure noise reduction, active Dlighting, or image authentication is on.
The numbers you give (15 amd 22 frames) sort-of-suggest some of the above, or that you used RAW + JPG.