I did do a few more tests. Limited to only copy to one directory, On the source (XP machine), generated 512 small testfiles of 1.09kByte in one directory and another directory with 256 files of 670kByte (just copy/paste of all the first file, two files, four files etc.)
Copying this time with drag/drop in two windows explorer windows, to a new directory on the NAS.
Copying 50 or so files is quick, until the directory already has a bunch of files in there. The copying itself delays, and than either goes quite quick, or starts that intermittent behavior: copy a few files, pause, and copy again a few files. I could not figure out when that happens.
So maybe it is not the amount/size of the files to copy, but what is already there??? Maybe in the subdirectories as well? (both gets large during copy large number of files/directories).
What might support that, if few 100 files are at the NAS directory, and with Ctrr-A (select all) and delete, it takes quite a while before the 'Delete confirm comes up.
Also copying the same files with overwrite slows down the NAS down even more.
This scenario is easy to create and reproduce.