In Linux, things that are automatically mounted usually have poor defaults for performance and no way to tweak the options to get better performance. I’m far from an OSMC “guy”, but I do know Linux.
The best solution for remote mounts is NFS.
Next best is CIFS (and it is about 30% worse than NFS).
The actual storage and file system used on the remote system matters greatly. FAT is slow. NTFS would be preferred if it is a Windows system with a spinning disk. Flash storage that isn’t SSD will always be significantly slower over the network.
If you have no choice except to use flash storage and FAT32/exFAT then direct connections is probably the best option provided you manually mount it and avoid gvfs/gio mounts. That means either using /etc/fstab or autofs to do the mounts. If the storage will always be connected and NEVER pulled, use the fstab. Otherwise, use autofs. The options and actual mounts are the same in both, but autofs is a little more complex to setup.
Of course, if the OSMC gurus here know that gvfs/gio aren’t used when mounting storage, then using these methods might not help at all. We need their guidance.