Cloud Dialogflow webhook authentication is done through one of the following three methods as mentioned here. Using token seems to be legacy or with Actions on Google, not strictly Cloud Dialogflow.
For the second query, I have come across this StackOverflow thread which has indicated that the 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.
Here a third party has provided a workaround on the above issue. Please note that it is not verified whether the workaround will work fine or not.
If still the issue is not resolved or If you have any technical queries or think your issue is related to code, you can post at the Stack Overflow or Server Fault and a community of developers will assist you.
In case there are any technical issues or you would like to request any feature you can report it at the Public Issues by selecting the right component here.
Please also note that Google Groups forum is meant for general discussion of the platform, not specific technical issues. You can find more info about Community Support Overview on Google Groups here.