That's rarely stock and many are trojans. The YouTube issue is all about the network. Even my old core2duo laptop with 4GB RAM and Intel 4500MHD graphics was OK pumping 1080p to the HDTV over HDMI so your machine is 10x that old laptop. I'd see if local videos play OK with VLC.
DotA at high settings does drop frame rate when a lot is going on. This is not new. I'm not duplicating the web on that. Try tweaking like the others do. Example.
http://aboutdota2.tumblr.com/post/106987558523/dota-2-ultimate-performance-guide-improve-fps
There are more for other games.
I recently bought a new computer after having some crappy computer from 2009.
I was really stoked to play all these games that my previous computer wasn't really capable of handling, however... I've gotten some issues playing video games.
Warframe runs like **** for me when there's a couple of enemies, which I find weird since my computer should be able to easily handle that game. The same goes for DotA 2 when a lot of stuff is happening, generally any game where a lot of stuff happens my computer begins to lag. The game stutters and I get weird audio cutouts.
I am able to run a game like MGSV fine without any lag (which I think is because not a lot happens around you??) but I can't even watch 720 60fps videos on YouTube without it stuttering.
I have no idea what could be causing this issue and much help is appreciated...
(Specs: http://i.imgur.com/VfAJX6M.png )

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