I have good confidence in Ernie's timings, note that the 10X lexar card does not seem any faster than the 8X card in the 990. It will be interesting to see more 10X card timings. Unlike the CP900 and CP950, the CP990 does seem to benefit from faster flash cards. The Lexar cards are not necessarily faster than others. The fact that cards show increased speed in the 990 might partly be due to the larger file size, as files get larger, the camera spends more time writing to the card, and flash card speed will become more important.
Larger cards do get slower as they fill up due to overhead in the directory probably.
My pet theory about this is that it has to do with the sequential file numbering feature. In the original CP950 1.0 firmware, the camera would overwrite files that already existed on the card with files in the sequential numbering sequence. After some complaints, this was fixed in more recent firmware releases. I suspect that what has to happen is that the camera has to check for duplicate file names on the card, and this gets slower and slower as you add more files. Plus add in the overhead of the larger directory, plus possible file fragmentation, and the operation gets slower as you add more files.