I do have some measurement on gemsun04 for Miriam's island with 500 SES.

   master::

            operation          |   tot_duration    | counts 
   ----------------------------+-------------------+--------
    computing ses              |  32748.4139366148 |   1000
    saving gmfs                |  10481.9452979557 |   1000
    computing gmfs             |  2430.58776307098 |   1000
    saving ses                 |  659.698407888368 |   1000
   
  fix-blocksize-conservative::

            operation          |   tot_duration    | counts 
   ----------------------------+-------------------+--------
    computing ses              |  32774.7693712709 |   1000
    saving gmfs                |  9106.89363884958 |    500
    computing gmfs             |  2440.51239848118 |    500
    reading ruptures          |  605.51239848118 |    500
    saving ses                 |  429.219392299639 |   1000
   
We are saving 1300 seconds in "saving gmfs" than more than compensate the 600 seconds spent in "reading ruptures". On a cluster we would save much more (the saving time of the GMFs in the cluster is 14 times the computing time).