About edge extracting process slow down strangely

Hi all,

I’m trying to extract edges from the oracle table using the oetl tool, and at first the speed retained about 2000 records/sec for quite a long time, but suddently fall significantly to several hundred records/sec and retained that speed or even slower since that, just as the following pictures show:

So I’m wondering what factor suddenly made the ETL process slow down significantly, since the server only had this process running. And what can I do to improve this situation?