there is a saying that the more accurate the information
the less room for questimation.
and that the answers speed and accuracy directly relate to the questions speed and accuracy.
post #13 and post #19 sum up the project.
post #14 has the essence of your answer.
one UNO sending the data into the cable
a second receiving and replaying. they both can be connected to a display to show % of signal loss or # of bits sent, speed etc.
you can send data, pictures, files, etc. when you look at the pictures you can send to an OLED, you can see how to store and send that data.
"USB to UART cable Supports +3.3V UART signals" this requires a 3v3 signal. I would recommend a 3v3 board and not a 5volt to 3 volt converter
Data Rates: 3MBaud
do you have a required rate ? or just testing to that rate ? do you need to push data faster to see when it fails ?