You should see a huge improvement on multi-GPU runs by enabling GPU direct communication. Please see my comments at Using multiple GPUs on one machine - User discussions - GROMACS forums (bioexcel.eu). Note that you should use the latest 2023 version of GROMACS, and build with thread-MPI (-DGMX_MPI=OFF). You can also use lib-MPI (-DGMX_MPI=ON) but it must be a CUDA-aware build of MPI to allow GPU direct communications.