IFeatureEdit.Split method looks more performance intensive then SplitAtDistance because after each split it stores new features, and deletes starting feature. And it looks to me you will call the Split function more then once for each Polyline.
Consider Neil advice and use IPolyline.SplitAtDistance. All operations are stored in local memory and you will store final features/geometries without continuous deleting in DB.
If you have large amount of long and complex features to process keep in mind that for complex feature Geometry at beginning of operation can take about 10Mb each. Just not to loose performance on memory paging and swap files.