MPI-IO codes can show decreased performance on Sulis using the default OpenMPI IO subsystem. Performance is improved by either switching to a different File Collective algorithm in ompio or switching to using ROMIO for output. This is easily done by placing one of the following export lines in you submission script
export OMPI_MCA_io=^ompio export OMPI_MCA_fcoll=two_phase
Do not put both export lines in your script, choose one or the other. You should test for which gives the better performance for your problem.