Closing a serial device takes one second

[Marc Balmer <marc%msys.ch@localhost>]

Closing a serial port (e.g. /dev/dtyXX) takes almost exactly one second.
 Why is that so?  Why does the call not return immediately?

int fd;

fd = open("/dev/dty03", O_RDWR);  /* returns immediately */
close(fd); /* returns after one second */