"Over the last few weeks, the performance of App Engine's Datastore has been seriously degraded. In addition to May 25th's 45 minute Datastore outage, applications have seen an increased latency and thus errors as a result of timeouts. There're a lot of different reasons for the problems over the last few weeks, but at the root of all of them is ultimately growing pains. Our service has grown 25% every two months for the past six months," explains Google.
"We want you to know we're taking current problems with Datastore very seriously. So until performance has returned to a state we're proud of, and that you're satisfied with, your apps willn't be charged for Datastore costs, for either storage or Datastore CPU effective on your May 31st bill. When we're convinced we've returned Datastore performance to a level we consider acceptable, we'll give you a 7-day notification before we begin charging again (Note: Your Datastore CPU budget will still control your app's CPU usage though you'll not be charged for Datastore CPU. So please don't turn your budget to $0 during this period)," noted Google.