Kara from the Google My Business team confirmed Google is working on a fix for the delays in getting review data from the Google My Business API, specifically webhooks. Kara said "Our teams are working on the issue but they don’t have an estimate for when the issue will be resolved."
The issue was reported in this Google My Business Help thread two weeks ago. Google finally responded the other day. The issue reported read:
I am seeing dealy in getting data from webhook(GCP notification). Please see the instance below.The review got created at 2018-12-13 11:48:09
but we got the notification only at 2018-12-14 04:41:15
This is the review details we received.
{ "type": "NEW_REVIEW", "locationName": "accounts/109109732438834244566/locations/9000166989250105070", "reviewName": "accounts/109109732438834244566/locations/9000166989250105070/reviews/KzQW9nWTh_hy2BCZnUdsVg" }
Kara said:
Based on what you've shared it appears it could be the result of a technical issue. Our teams are working on the issue but they don’t have an estimate for when the issue will be resolved.
I am not sure how many noticed this issue but Google is finally aware of it and working on fixing it.
Forum discussion at Google My Business Help.