Putting that directory in RAM or using memcached might help. If you run your Zammad instance on spinning disks, this will probably have some sort of positive impact on performance regarding disk I/O. We use SSD’s in our setups, so for us there’s little performance gain of having that data either in RAM or on disk.
You may want to check out some of the troubleshooting tips mentioned here (and read the warnings on blindly following advice):
You might also look into tuning PostgreSQL, if you haven’t already. You can compare some configurations here: