so you want to interpolate between the numbers, or fill in the blanks so to speak. Your sensor data is not going to be 'slowish' if it is coming in at 500hz, whcih is far apst he threshold of perception... but from what you're saying, the data jumps around?
Since CK is not here yet I will take shot at the answer and say that you have lots of clock cycles left after getting the sensor data on SPi to be able to blast it out over the serial port between sense data-taking.
//some code here
//Average the data
//some more code here
SensorDataTakingFunction(); // get slow sensor data
AverageThatData(); // average the accumulated data