While I'm not an MS Access pro, nor am I an expert with regard to document scanning technologies, I have had some experience with large scale document scanning.
Per what I read in the link provided, I'm assuming that you're using a "run of the mill" desktop flatbed scanner to accomplish your goal (I, of course, could be wrong) - and this software seems to be geared toward the type of use that those types of scanners are built to handle.
Compared to something thats' officially classified as a "high speed scanner", the average desktop scanner and accompanying software packages aren't going to operate very quickly - and the software is often a little resource heavy.
The high-speed stuff I've worked with have been simultaneous duplexing high speed scanners made by Fuji that scan up to 65 documents per minute.The software package was this thing made by Captiva that did all sorts of cool stuff. They had the software instlled on a computer that didn't do anything other than dealing with the scanned documents. The company used it for expedited processing of medical claims.
his could work for you - have your scanning software on a computer that does nothing but deal with the scanned stuff (including OCR and/or PDF stuff) and have that computer feed the files it makes to a storage space and ahve acess visit that storage space. This should speed things up.