More extensively, the structure looks like this:
volatile tx_buffer_index_t _tx_buffer_head;
volatile tx_buffer_index_t _tx_buffer_tail;
// Don't put any members after these buffers, since only the first
// 32 bytes of this struct can be accessed quickly using the ldd
unsigned char _rx_buffer[SERIAL_RX_BUFFER_SIZE];
unsigned char _tx_buffer[SERIAL_TX_BUFFER_SIZE];
The comment means that the HypothethicalNewVariable I added would be slower to access than the variables before the buffers, because of the limitations of LDD that John explained.
It's pretty stupid; the Arduino Serial code isn't very well optimized ANYWAY, and the extra cycles to access some new variables in the "wrong" place are pretty meaningless in the grand scheme of things.
Still, it'd be better to add new variables BEFORE the buffers, if you have to add new variables (say, to support HW flow control or RS485 or something.)