As my opinion, Reading and parsing excel must load whole excel into memory and take much memory to parse excel, so please take care of the memory usage, it might be a killer of your application, you know as default setting in php.ini, there are just few memory can be used, try to add this code at top of the code snippet and see how does it go:
ini_set('memory_limit', '10240M');#Memory usage, assume you have enough memory to use.