YouTube loading slow?

Hello, what could be the case that on FortiGate 140D YouTube service is running slow. I mean, something is in inspection to deep? I've set up a policy WAN->LAN, LAN->WAN with AV-flow, web-filter-flow, block-p2p, ips_all_default - are there UTM features related that YouTube videos could load very slow? (It is a 40Mbps bandwidth with BGP, any other site is loaded like thunder). Where I should search for diagnosis?