It was part of the outline of a load-balancing mechanism, but it's not currently used. The idea was that if you're running more than one task per host in the virtual machine, the faster machines should get more tasks. The scheduler used by pvm_spawn() should look at the speeds and current loads when assigning new tasks to hosts. You can actually set the speed of each host (which defaults to 1000) to a value from by using the host file sp= option.
It's possible to write a wrapper function for spawn that reads the host configuration and list of running tasks and makes such a decision. If you're really serious about it, think about writing a resource manager.