This event is emitted when AJAX requests
are taking long to finish loading.
By default Unpoly will wait 400 ms for an AJAX request to finish
before emitting up:network:late. You may configure this delay like this:
up.network.config.badResponseTime = 1000 // milliseconds
Once all responses have been received, an up:network:recover
will be emitted.
Important
If additional requests are made while Unpoly is already busy waiting,
no additional up:network:late events will be emitted.
Also see Loading indicators.
