It would be I think: hard coded into the viewer program.
On the iLiad I had coded an extension for a file you could create/delete that turned on the fast erase/slow erase for the PDF viewer (iRex provided no access to add configuration items.)
It is also the case that the erase timing could be made programmable. Some people like faster page flips and are not annoyed as much with the ghost effect. While others want no ghosts and are happy to trade off a slower page flip to get it. And some want it just right: a little slower, a little less ghost.
