Traditional zero clients do not support RTAV and it can only be used with thin clients that support it or with the Horizon Client. By default with RTAV the webcam will be restricted to 320 x 240 and 15 frames per second. You can increase this up to a maximum of 1920 x 1080 and 25 frames per second but it comes at a cost of resources. It's all about finding the right balance.
Setting Frame Rates and Resolution for Real-Time Audio-Video on Horizon View Clients (2053644)