Unused memory on Linux will be automatically used to cache IO operations, and you can also tweak PG itself to use more memory during queries (search for "work_mem", though there are others).

If your workload is read-heavy, just giving it more memory so that the majority of your dataset is always in the kernel IO cache will give you an immediate performance boost, without even having to tweak PG's config (though that might help even further). This won't transfer to writes - those still require an actual, uncached IO operation to complete (unless you want to put your data at risk, in which case there are parameters that can be used to override that).

For write-heavy workloads, you will need to upgrade IO; there's no way around the "provisioned IOPS" disks.