In this case, a customer reported low CPU usage and long render times for a pass that had many framebuffers.

After some investigation, we found that disabling On-Disk Framebuffers (mental ray settings, Framebuffer tab, under Advanced Settings) made a big difference:

// Disable On-Disk Framebuffers so framebuffers are stored in memory
SetValue("Passes.mentalray.VirtualFramebuffers", false, null);
OpenView("Render Preview", null);
// INFO : RC   0.4  info : wallclock  0:00:07.27 for rendering


// Enable On-Disk Framebuffers
// Slows down the render, and CPU usage is much lower on all cores
SetValue("Passes.mentalray.VirtualFramebuffers", true, null);
OpenView("Render Preview", null);
// INFO : RC   0.4  info : wallclock  0:01:15.31 for rendering