>Is it because of the number of files in the directory?
>Is there a limitation?

Old issue that keeps coming up...
Conventional directories are simply flat, sequential files which contain
a list of file names which cross-reference to the physical data on the
disk.  As primitive data structures, directories impede performance, as
lookups are serial, take time, and involve lockouts as the directory may
be updated.  As everyone finds, on multiple operating systems, the more
files you have in a directory, the worse the performance for anything in
your operating system going after files in that directory.

The gross rule of thumb is that about 1000 files is about all that is
realistic in a directory.  Use subdirectories to create a topology which
is akin to an equilateral triangle for best performance.
     Richard Sims, BU