Pre-fetching may be interesting if the first SQL query using prefetched data picks a small set of data stored on a disk plate here, then another one stored far away (distant cylinder), then back, then again far away, and so on... On a mechanical HDD the implied head movements may considerably slow this first query down.

That's extremely common for index based accesses. Which in turn are a large fraction of queries in most use cases.

If I'm right pg_prewarm if not adequate with a SSD storage device,

For SSDs random accesses and small accesses are still substantially slower than bulk reading data into memory.