Node Save is Very Slow for Large Site

My site has 350K in {term_data}, 1 Million in {term_node}, and 240k in {nodes}. Now, when i insert or update a node, the form takes around 30 sec to save the data. I tried to disable the content of node_submit in the node module, but the save operation is still slow.

I appreciate if someone could point me out what mysql operations could be causing that delay, or the sequence of functions that are called when the form submit button is clicked.

Thanks!