Sorry, I didn't get as far with the user accounts as I intended to. Possibly as a result of redirecting the domain name over, Google's search engine bot decided to index the site. Normally, this isn't a problem as it just grabs a few things but I think it saw major changes and decided to go nuts.
There is a setting for webmasters to control how fast it will crawl a site which I had set. However, if they detect that the site is running on a Content Delivery Network (which the site now is) then the limits are discarded and they go full speed. The Google spider went berserk - it's possibly a direct link over their own network without having to go out onto the interwebs so it could go fast, really fast - several hundred requests per second. Because of the extra traffic the cloud service launched extra instances to satisfy the requests.
App engine runs on what's called "container" technology. Like a virtual machine, it's a little instance to serve your site. Instead of the 16Gb server the old site had been running on it now runs on either a 128Mb (yes, Mb!) or more instance. Your phone has more memory than the server that now runs JWD :)
For normal use it has 1, 2 or a few instances running throughout the day as demand fluctuates. When the search engine hit it there were 80+ instances and I didn't want to use the DoS protection to block the Google index - I want the site indexed, I just needed to control how fast it went. Sadly, Google don't honour the settings in robots.txt to set the crawl rate so I had to cobble together a rate-limiting throttle to send 429 responses to Google (a webservers way of saying "whoah, slow down pal!").
That worked but it took a lot of the time I was intending to spend on the user sign-in issues. It's also why some people may have seen a "quota exceeded" message for a short period as the site went over the billing limit I set.
One the plus side, we can cope with a lot of traffic if we need to and while it may have made things slightly slower, overall it coped very well and wasn't too noticeable.
Go cloud !