All of the computation, including pre-processing, is offloaded to the TPU. The weak machine is really just idling. A bigger one will only cost money and have no measurable effect on the performance.
This is why my previous comment mentioned that GCP is a better benchmark for this since you can select the number of CPUs to match with the GPUs to some extent. You can get a rough idea of the savings by looking at their P100 instances.