The script calls the ReconfigVM_Task method.
Each SDK method is mapped to 2 entry points in PowerCLI, the method with a _Task suffix and without that suffix.
If you call the method with _Task suffix it will return immediatly, the entry point without the suffix will wait till the task is complete.
So in fact, the script above should already be launching these calls without waiting for the completion.
Perhaps you are hitting a vSphere limitation on the numbers of tasks that can be running in parallel.
How many of these tasks do you see running in parallel, if any ?