LSM9DS1 breakout - How to increase speed
Check the header of the library, 

you will find readAccel readMag readGyro and readTemp in separate public functions and looking at the implementation of read it just calls those 4 in sequence. readAccel will update the accelData member field.

So you can just call lsm.readAccel() in the loop and then grab lsm.accelData directly and ignore the rest.

void loop() 
{
  lsm.readAccel();
  accelX = lsm.accelData.x;
  accelY = lsm.accelData.y;
  accelZ = lsm.accelData.z;
  rms = (accelX*accelX)+(accelY*accelY)+(accelZ*accelZ);
  Serial.println(rms);
}