You think VLC pulls the VP8/9 version instead of the h264 version? I seriously doubt it. Why would it? It would make no sense. Most devices have h264 hardware decoders. Most don't have VP8/9 decoders (though they exist).
I also have no doubts that an Athlon64 could decode 720p h264 completely in software. You'd be pegging that single core pretty high, but it could do it.
My point was that the hardware acceleration, and vp8 vs h264, doesn't really matter. The computation power of processors sold in 2010, even those in not-very-thick laptops, was probably double my Athlon64. The difference is everything else, the UI stuff (and maybe also flash and browsers being less efficient at decoding than ffmpeg).
Well, on a single core you are going to hit issues with multiple processes running. If you have the browser, flash, network, and decoding all happening on a single core, it wouldn't be surprising to see stuttering. Obviously, offloading the decode to a different processor would help out, but so would running just a single process.
I also have no doubts that an Athlon64 could decode 720p h264 completely in software. You'd be pegging that single core pretty high, but it could do it.