WARNING| Rendering utilization was only 48%. Your render may be bound by a single threaded process or I/O.

In general, if you see this warning about low machine utilization, that means that a significant amount of time is being spent in just one thread, and if you want to render faster, you need to look into possible issues.

• Reading textures (file I/O) isn’t multi-threaded, so that can reduce machine usage because threads will be waiting while textures are read from disk.Check the amount of texture data “Read from disk” and the “File I/O time”, and compare the amount of texture data with the texture cache size (for example, if you’re reading 5GB of texture data but the cache is just 1GB, you can increase the cache size).
• Subdivision isn’t multi-threaded yet, so a few huge objects might slow things down. But in general, multiple objects will be subdivided at the same
  time (one thread per object) so it isn’t usually a big issue.
• Loading standins is also file I/O so each file is loaded in a single thread. File I/O is dependent on the file system, not the CPU speeds, so adding
  more threads to IO won’t help. If this is what is causing your low CPU utilization, the only way to improve that is to get faster disks, file
  servers, networks, etc…
• Displacement is multi-threaded. A single mesh can be displaced by multiple threads and multiple meshes can be displaced at the same time.
• Another possible reason for this warning is because there are multiple copies of Arnold running, or Arnold and some other compute-heavy application, so that Arnold can’t get to 100% utilization because Arnold is competing with other processes. You can check your Activity Manager (OSX), Task Manager (Windows), or top (Linux) to see see what else is consuming CPU time.
• You’ll also see the utilization warning with relatively simple scenes, because the single-threaded work of loading the scene and plugins and textures
  takes longer than the actual rendering.

hat tip: Thiago