Normally I can shoot 9 JPEG's into the buffer. This amount is indicated by the number in the lower right of the viewfinder, next to the focus confirmation light.
Now that number says 2. And indeed, if I shoot more than two JPEG frames subsequently, the camera will hang to write to the SD card and then will proceed shooting further.
This behaviour is independent of SD card speed, JPEG size or quality, or shooting mode.
When I switch to RAW I can shoot 4 frames in a row.
It seems that my camera buffer is partially filled, but I can't find out how to empty it.