Unfortunately these things can occur in the darndest of places, as a bug in Google CLoud, an incident at GCS, or maybe even in your monitoring stack. I would encourage you to hold off judgement until root cause is identified.

One assurance I can make is that Google SRE monitors these things very carefully 24/7, and such levels of latency in the service would be treated as an incident. So it's likely something else is going on.

(work at Google Cloud, but not on GCS or support)