Correct, according to that article, the Sony A9 is doing some kind of processing to the JPEG files which adds CPU overhead, thus causing slower write speeds.
I am guessing the RAW to JPEG conversion is what's making the JPEG write speed slower than just writing RAW to SD.
Continuous Hi
Compressed RAW
0.05 second
(20.00 fps);
240 frames total;
45 seconds to clear*
Time per shot, averaged over 240 frames, then slowed to an average of about 0.29s or 3.40 fps when buffer was full with a lot of variation.
Continuous Hi
Compressed RAW + L/F JPEG
0.05 second
(20.00 fps);
225 frames total;
87 seconds to clear*
Time per shot, averaged over 225 frames, then slowed to an average of about 0.40s or 2.50 fps when buffer was full with a lot of variation.
* Using Lexar 2000x SD Card