Also, a 12daz is a zipped 12da, so there is an extra step involved where the data is extracted before being read in. Much like in any other zip archive, larger file = longer to extract.
Where are the files stored? Local or network drive?
It was only a few strings in 12da files but it turned out that problem was the folder that we save our field files on our server which now has to go through a lengthy scanning process every time we read or copy from it.(a new IT procedure)
Thanks for your answer anyway.