There is a period of time between the moment App Engine decides it needs a new
instance and the moment the instance is available to handle the next request off the
request queue. During this time, App Engine initializes the instance on the server
hardware, sets up the runtime environment, and makes the app files available to the
instance. App Engine takes this preparation period into account when scheduling
request handlers and instance creation.
The goal is to make the instance as ready as possible prior to handling the first
request, so when the request handler begins, the user only waits on the request handler
logic, not the initialization. But App Engine can only do so much on its own.
Many initialization tasks are specific to your application code.
App-specific initialization potentially puts undue burden on the first request handler
to execute on a fresh instance. A “loading request” typically takes longer to execute
than subsequent requests handled by the same instance. This is common enough that
App Engine will add a log message automatically when a request is the first request
for an instance, so you can detect a correlation between performance issues and app
initialization.
You can mitigate the impact of app initialization with a feature called warmup
requests. With warmup requests enabled, App Engine will attempt to issue a request
to a specific warmup URL immediately following the creation of a new instance. You
can associate a warmup request handler with this URL to perform initialization tasks
that are better performed outside of a user-facing request handler.
To enable warmup requests, activate the warmup inbound service in your app configuration.
(Refer to “Inbound Services” on page 76.) Set this in your appengine-web.xml
file:
<inbound-services>
<service>warmup</service>
</inbound-services>

Warmup requests are issued to this URL path:
/_ah/warmup
You bind your warmup request handler to this URL path in the usual way.

There are a few rare cases where an instance will not receive a
warmup request prior to the first user request even with warmup
requests enabled. Make sure your user request handler code does
not depend on the warmup request handler having already been
called on the instance.