How and when to use /dev/shm for efficiency?
You don't use /dev/shm. It exists so that the POSIX C library can provide shared memory support via the POSIX API. Not so you can poke at stuff in there.
If you want an in-memory filesystem of your very own, you can mount one wherever you want it.
mount -t tmpfs tmpfs /mnt/tmp, for example.
A Linux tmpfs is a temporary filesystem that only exists in RAM. It is implemented by having a file cache without any disk storage behind it. It will write its contents into the swap file under memory pressure. If you didn't want the swapfile you can use a ramfs.
I don't know where you got the idea of using /dev/shm for efficiency in reading files, because that isn't what it does at all.
Maybe you were thinking of using memory mapping, via the mmap system call?
Read this answer here: https://superuser.com/a/1030777/4642 it covers a lot of tmpfs information.