Hey Peter,
Looking at your project, I do indeed see the latency spike in your App Engine graphs. This has proven to be transient as App Engine was able to recover back to its normal latency levels as you pointed out. There are multiple reasons why latency could occur, such as networking issues between the client and Google and/or within App Engine.
If you see a latency spike like this one that persists for a longer period in the future, I ask that you open a
Public Issue Tracker with the time of when the spike occurred along with your project ID and service (aka module) that experiences the latency, and our engineering team will be able to asses the impact; as Groups is strictly for general product discussions.