It slows down everything (profiling shows 10-15% of time elapsed in this function) and can easily be replaced:

eZDir::path( array( $this->RootDir, $this->FileName ) );

can just be:

$this->RootDir . "/" . $this->FileName;

PHP plays very well with / even on Windows OS.
We shouldn't care about a path being ended by a dir separator or not, if unsure => just add it. It is safe to use multiple dir separators like in: "var//cache///"