Right. My point is that while spawning bgworkers probably helps, I don't expect it to be enough to fill the I/O queues on modern storage systems. Even if you start say 16 prefetch bgworkers, that's not going to be enough for large arrays or SSDs. Those typically need way more than 16 requests in the queue.
Consider for example [1] from 2014 where Merlin reported how S3500 (Intel SATA SSD) behaves with different effective_io_concurrency values:
Clearly, you need to prefetch 32/64 blocks or so. Consider you may have multiple such devices in a single RAID array, and that this device is from 2014 (and newer flash devices likely need even deeper queues).'
For reference, a typical datacenter SSD needs a queue depth of 128 to saturate a single device. [1] Multiply that appropriately for RAID arrays.
