Ifort and gfortran/OpenCoarrays do both show the same low coarray performance (pattern) with (Intel) MPICH. To my current understanding, this is related to MPICH’s way to configure for use with mismatching arguments with the Fortran compilers (an MPICH requirement for it’s functions with void?).
Starting with gfortran release 10.0.0 argument mismatches are detected differently with the compiler and MPICH must be configured accordingly. To my current understanding, this new way to configure MPICH with the Fortran compilers leads to a very low coarray performance pattern with gfortran. (I do observe the exactly same low coarray performance pattern with ifort and Intel MPICH). I did try out recent gfortran (with OpenCoarrays) with different MPICH versions, resulting into the same poor coarray performance.
A simple trick to achieve high coarray performance with MPICH is to use older Fortran compiler releases: Gfortran releases prior to 10.0.0; prior ifort releases did also work for much higher coarray performance (I just don’t recall before which ifort version it was).