how to handle large data?

I'm tyring to import a large XML (~23mb) via REST-API to my CMS ..
I read the XML via simpleXML -> process and validate the data -> preform a api-call for each node ..
everything semms to work fine on a smaller test-XML.. but when i switch to the final XML I like to import, I sooner or later get a 500 Server-Error after some hundred lines.. seems like the webserver isn't able to handle so many requests..

I already set max_execution_time and memory_size to very high values, but with no effort..

has somebody experience with similar problems? .. what can I do, to make it work?