A lock file is created on every hit (private files)

When using ImageCache with private files, _imagecache_cache() is invoked for all generated images, even though the generated file already exists.

_imagecache_cache() generates a lock file. This is a somewhat expensive an unnecessary operation.

I suggest checking for the presence of $dst as early as possible, possible even before the is_file($src) check (unless you want to prevent access to generated files after the source image has been deleted), and only use a lock file when generating images.