Well, I presume that it's because there's a lot going on in ONS mode that the framerates are low, but in the modes I usually play, I saw rates of 100 - 120 constantly (in BR and Instagib modes).
Can't wait to try ONS mode more once I understand it. By the way, in this slew of tests the odd strobing was mild at first, then got less and less until it went away completely. Weird. No crashes at all, touch wood.