Just read through the benchmark stuff you posted. Games and CAD packages use complex arithmetic functions to calculate how each pixel on the screen should be displayed. That isn't required for video streaming. The computer is just displaying images that have already been calculated in advance when the film was digitally encoded.
Movie playback from the Internet can be subject to stuttering depending in your connection speed. If you are using wifi to connect to your router, try a direct cable connection to the router if possible and see if the pauses still occur. Also worth trying streaming video from a few different sites at different times of day. The delays may be occuring at the servers and not on your computer at all. It all depends on how many users they are having to service at once.
The benchmark figures for the office and desktop type stuff are not brilliant, but they are about what you would expect from a six year old computer that is being compared against today's latest and greatest machines. It isn't doing too bad, all things considered.