"Emulate async read" parameter

[sundance2]

I'm trying to understand the impact of the advanced parameter "Emulate async read" when loading games with Dreamshell.
(Note : on Retrodream, this parameters seems to be (mis?)named SYNC).

On Dreamshell, allowed values are "none", 1, 2, 3, 4, 5, 6, 7, 8 and 16.
On Retrodream, they are called "TRUE", 1, 2, 3, 4, 5, 6, 7, 8 and 16.
So basically the same, just a name change.

When testing different parameters to load a game, one can find many (different!) advises here and there about which value to select, and these values vary per title.
It's unclear to me why 8 should be preferable to 16, or 2, or "none",

For reference, here is the information I could collect so far, note though that they could be false :
- Higher async values are supposed to improve read performance
- "none" is supposed to be the best for compatibility, but worse for performance.
- Unfortunately, performance itself can be a problem for compatibility, so "none" is not a sure-win.
- Higher async values are supposed to be "worse" for compatibility, though I'm not sure to understand in which ways ?
- I also don't know if this value impact the resident memory size of the loader, which could then impact compatibility. For example, does a higher async value means a larger look-ahead buffer ?
- It's also unclear to me if storage fragmentation can be a problem for this setting.
- The impact of this setting is said to vary depending on loader version (?)

Based on above explanations, it seems that it would be better to target 16 whenever possible, for better performance, and scale down from there, whenever it seems to hinder compatibility ?

Rather than relying on urban legends, I would prefer to read here a good explanation of the parameter, to get a good sense. Therefore, could it be possible to explain the expected outcome of this parameter ?