Response time of RTS and DTS pins of the chip FT232RL (USB to Serial)
There isn't a definitive upper limit to the latency. The OS may delay running your process or running the FTDI driver or sending the USB packets randomly. If e.g. another USB device on the same bus consumes a lot of bandwidth, the FTDI packets may experience a long delay. If other CPU-intensive processes are running, your program might not even be scheduled quick enough. Python might decide to do some GC and block your program when you want to send your data. To get exact timing you need to use an RTOS and a different language anyways. In any case, 16ms sounds very optimistic.
Basically, an ordinary desktop OS (Windows, Linux without RT stuff) plus Python plus USB is probably the worst combination for real-time applications.