QOS

Hi. QoS is an "end to end" thing. It may have little or no effect to just enable it. Have you identified that packet loss occur in the firewall or neighbouring interfaces (queue drops)?
A regular traffic shaper in the firewall may be best solution if congestion occur here. Use Internet service database if possible to map traffic. /C

Hi QOS,

Have you tried creating a policy using Google and Microsoft Skype from the ISDB in the destination and placing the policy on the top?

Also, you can check from the debug flow if are seeing any drops on the FortiGate interfaces for the concerned traffic.

You can use the following link to see how to debug packet flow on Fortigate.

https://docs.fortinet.com/document/fortigate/6.2.3/cookbook/54688/debugging-the-packet-flow