to increase the reading and writing/printing speed of data from Arduino Nano to Raspberry Pi 3B+
parseInt reads digits and stops after a non-digit characters occurs or the function times out because no next characters arrives. The timeout is at default 1 second. So this is where you loose time.

Simplest solution for your code is to send some non-digit character after the number sent from Raspberry. This will terminate parseInt immediately. Usual is to send a new line character. It will separate the numbers nice if you debug the communication with prints to serial terminal.

Not a very good option, but I mention it, is to set the timeout of blocking read functions like the parseInt to a smaller time with Serial.setTimeout(ms).

To really speed up the communication you would send the numbers in binary form and not as text like you do now.