This may be affecting other builders, but I noticed it on chell-incremental.

https://luci-logdog.appspot.com/v/?s=chromeos%2Fbb%2Fchromeos%2Fchell-incremental%2F825%2F%2B%2Frecipes%2Fsteps%2FVMTest__attempt_2_%2F0%2Fstdout

That output contains a steady stream of:

14:39:46: INFO: RetriableHttp: attempt 5 receiving status 504, final attempt
14:39:47: WARNING: HttpsMonitor.send received status 504: {
  "error": {
    "code": 504,
    "message": "Deadline expired before operation could complete.",
    "status": "DEADLINE_EXCEEDED"
  }
}