Why does /clock slow down under 65% CPU load?

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On Ubuntu 16.04, with 8 cores and lots of nodelets.

Measuring /clock with rostopic hz /clock and using rosbag play --rate xx, I see that:

--rate    CPU load    /clock
  0.5        40%       50 Hz
  1.0        65%       38 Hz
  2.0        85%        9 Hz

I know ROS doesn't claim to be real-time, but normal-speed playback shouldn't be slowing down the clock as the CPU has plenty of cycles to spare, it appears.

This affects running the program live, as we have a ros::Timer() that slows down, mucking up everything else down the chain. Using a stop-watch, at --rate 2.0 the BAG file plays back at double speed, but why isn't /clock keeping up?