Using EWS, CIC sends a reqest to Exchange to query the voicemails in the user's Inbox, specifically by having Exchange filter for items of type IPM.Note.I3Vox. Filtering on the Exchange side rather than within CIC generally improves performance.
What you can do is filter the EWS Connector log (turned up to All) for an EwsRequest containing the string IPM.Note.I3Vox, with the Mailbox attribute filtered for the user's email address. Bookmark that EWS Request (the EwsRequest log entry itself and not the EwsRequestHttpsHeaders) and make a note of the thread it was sent on. Unfilter the log and then filter it on that thread and find the first EwsResponse on that thread after the bookmarked request. That is the response to the bookmarked request.
If you look at the time gap between them, you can see what the round-trip time was for that request going to Exchange and the response coming back. Generally for an on-premise Exchange server, we'd want to see that in the order of 50-100ms, dependent on your architecture. For O365, the response time is typically longer, but typically we'd like to see a few hundred ms and certainly not a response time measured in seconds.