Charlie I did some reading previous to posting the question.
We don't have a crystal ball, and your question didn't reveal any prior research.
After transferring some files to /dev/shm and doing some speed test. I would of expected those files in /dev/shm to be much faster that being placed in regular directory.
Maybe linux is swapping those files in memory(whatever files where placed in /dev/shm) to disk.
Linux always uses memory for disk caching. It's called the buffer cache.
Whatever performance problem you are trying to deal with can probably be better addressed some other way than you trying to change tmpfs or /dev/shm settings.
I try to keep things standard as can be.
Good. Don't change /etc/fstab. You might break something, and it's unlikely to solve your problem.