Not sure about design table but check Document Manager as that can get you the file properties without opening them OR try using DSOFile option.
Please not that for using document manager, you need to obtain a key from SOLIDWORKS. If you need a key then please send a request to SOLIDWORKS.
Just wanted to point out... Please consider that DSOFile only gets the general custom properties and not the configuration specific ones.
You could also open the file silent and in view mode in SolidWorks. Not sure what you can access in that mode but it might save time since it won't rebuild. I would definitely investigate that option further. Another idea is to turn of rebuild in SolidWorks before batch run to save time...