This is what i received from Dialogflow support.
Hi,
Thanks for reaching out to Dialogflow Support. Webhook timeout limit
for Actions on Google integration is 10 seconds. For all other
integrations, including self-developed implementations sending
requests to our API, webhook timeout is 5 seconds. These values are
not customizable. The timeout limit includes time for Dialogflow
requests to your webhook endpoint, the webhook processing time, and
webhook response time back to Dialogflow.
Conversational interfaces are meant to be designed as a continuous
message exchange between the end user and the app/bot. If your web
service requires more time for executing operations in the background
and this cannot be optimized, consider redesigning the conversation
flow in such way that end users don't wait for the app/bot reply for
more than 5 seconds (10 for Actions on Google).
Let me know if you have any questions.
Regards, Mark Dialogflow Support Team
So the only option is to keep user in conversation while you receive the response from webhook