The current experience of mine is that it takes 1-3 ms to log data but sometimes it takes 0.2 seconds!!
When you call File::print(), the print() method checks to see if there is room in the buffer for the data to be printed. If there is, the data is added to the buffer, and print() returns, nearly immediately.
If there is NOT room, then the contents of the buffer need to be copied to the hardware. THAT takes a lot longer.
This I don't understand. If it is SPI-based, why it should wait for SD card to acknowledge
Why do you assume that such a thing IS happening?