The problem is this:
byte size = Serial.readBytes(input, INPUT_SIZE);
You are asking readBytes to read 30 characters, which it will try to do. If it doesn't read 30 chars it will timeout after one second because you didn't tell it to do anything different.
If the data being sent end with a linefeed, you would be better off using:
byte size = Serial.readBytesUntil(,'\n',input, INPUT_SIZE);
but you must make sure that the line is no more than 30 characters long including the linefeed.