Title says it all 
I’m using a 3.3v mcu and 5v display which uses serial port to communicate.
I don’t think 921600 baud rate will hamper the performance of SparkFun Logic Level Converter - Bi-Directional. It just converts the voltage level. Let’s see what Team Sparkfun has to say.
ping
Been awhile since I played with one of these but i seem to remember them topping out at 9600 or 19200 baud. You’d need a different converter for high speed data.