My problem is the same as written above. So I believe the solution (I found somewhere in the net, I'm sorry I haven't posted it all at once) will be of help to me too.
The version of firmware for my MFU is 061.121.224.18803. For the moment it's the latest as far as it's stated here http://www.support.xerox.com/support/workcentre-7545-7556/file-download/enus.html?operatingSystem=macosx&fileLanguage=en&&associatedProduct=XRIP_WC7525_base&contentId=114521&from=downloads&viewArchived=false
The older firmware doesn't have the problem of slow scanning as it's stated above. Maybe any alternative firmware exists, I don't know.