What are the rest of your specs?
Also, multiplayer BF4 should be using way more than 30% of your CPU, most large multiplayer games become CPU limited especially in larger matches as there is so much going on, but the GPU load has remained the same because your field of view is the same. I also don't think your skyrim number is right either, given that my PII 955 is the limiting factor in skyrim for me, unless you have settings turned way down or checked while you were in a tiny house or something that seems wrong as well.
As for the game in question, flash doesn't have much GPU acceleration support so flash games require a lot of work to be done by the CPU, the graphics may not be intense but flash is not an efficient language and is very bloated, turned the first starcraft game into a flash game you would probably still run into hardware restrictions simply because flash is bad, this is why more and more places have been moving away from flash for videos as there are better options out there, and there are better options being worked on for interactive web games so that we can finally kill off flash and all its bloat and security issues.