And you've got your settings.json entered correctly with the like, 3 entries for OpLog?
How many entries are you putting in?
SCO as I linked before is pretty hardcore on CPU.
My OpLog server is now 2 cores, 16 gb ram. My Node server is only 1 CPU, 4 gb ram.
The amount of optimization I've had to do is staggering. I'm returning 30 documents with 20 fields each. Managing their subscription and publication is something that's JUST as important as oplog.
Where do YOU think the weak point is?