We did some testing on full linux setup (linux server, linux client) and we found out that nfs4 can have much worse performance than nfs3 ... and that's even common knowledge (or perhaps not so much, it did surprise us). I don't know if it's possible to select NFSv3 when setting up NFS client in ROS, but you may want to try that. NFSv4 comes with some additional functionality (when compared to NFSv3) and if you don't require that functionality, NFSv3 might be better choice.