This will be very difficult to troubleshoot via a forum tbh.
The 3 second wait for the icon to become available is fine and quite normal. I'd expect the 3 seconds to be quite the same for when you're users are internal. This just indicates that the client has registered with the SIP Proxy Registrar.
Are you saying that the user has to wait a further 2 minutes once the icon has appeared before
initiating AV or are you saying that it takes 2 minutes for AV to work after
The problem seems to be the TURN server as this is used for external connections and not internal. For just a couple of users I'd expect a low spec machine to work fine. If you have logging enabled on the TURN server this can greatly slow it down so please check and remove all logging and restart and then check again. I banged my head for 30 minutes the last time I left verbose logging on wondering why it wasn't working and it wasn't until I saw the CPU maxing out that I thought about logging.
OK so are you using UDP or TCP (or both) for TURN and are you using it over 3478 or say 443?
Do you have TURN server authentication enabled?
Are you using SIPS and TLS as per https://www.ibm.com/support/knowledgecenter/SSKTXQ_9.0.0/admin/secure/sec_av_transport_tls.html
Enable client trace and look for ICE timeouts