The CP210x is a Full-speed USB peripheral based on the USB-Bulk transfer protocol. The CP210x is not a standard USB class device because it provides functionality that is not common (virtual COM port), so it requires a special driver to be installed (does not use one of the standard USB Class drivers, like Mass Storage and HID).
From the PC to the end device:
The CP210x driver sends 64-byte packets until the CP210x transmit buffer is full. The driver will then wait until the CP210x has room for another 64 bytes and send another packet. The USB protocol also has framing every 1 ms, so an example transfer from the PC to the CP210x is as follows:
t = 0 ms 64 bytes sent to CP210x, CP210x ACKs 64 bytes sent to CP210x, CP210x ACKs 64 bytes sent to CP210x, CP210x ACKs ... (until CP210x buffer is full) 64 bytes sent to CP210x, CP210x NACKs (no room)
t = 1 ms 64 bytes sent to CP210x, CP210x NACKs (no room)
t = 2 ms 64 bytes sent to CP210x, CP210x NACKs (no room)
t = 3 ms 64 bytes sent to CP210x, CP210x ACKs 64 bytes sent to CP210x, CP210x NACKs (no room)
t = 4 ms
and so on
The CP210x will transfer the data from its internal buffer to the end device at the appropriate baud rate and settings (handshaking, data bits, etc).
From the end device to the PC:
The CP210x will wait to send data to the PC until its receive buffer is full or the UART Receive Timeout has expired, which varies depending on the CP210x device. With some CP210x devices, the UART Receive Timeout can be adjusted. Please refer to AN205 'CP210x Baud Rate Support' and the appropriate CP210x datasheet for more information. Application notes can be downloaded from the Silicon Labs Applications webpage: https://www.silabs.com/products/mcu/Pages/ApplicationNotes.aspx.
Because of the added USB overhead, the VCP drivers and CP210x will always have more latency than a real COM port transmitting at the same baud rate, as shown below:
CP210x:
Real COM port: The timing of the CP210x also differs greatly from a real COM port, as the CP210x is designed for burst transmission and the real COM port will always be timed UART transmission.
CP210x USB Communication