There should not be any difference between an UNO and a MEGA in terms of how long it takes to execute a Serial print.
How long it takes for Serial.print() to return is dependent on the baud rate and the amount being printed. There is a 64-byte buffer, as long as the buffer will hold whatever is being printed, print() just places the characters in the buffer and returns. If the buffer is full, print() waits for space to become available, and returns as soon as the last character is placed in the buffer.