Playback on base mini a bit laggy

Hi all,
I have a base Mac mini, with 4GB of RAM and no upgrades (see signature). I don't use photoshop or heavy applications but I watched a 2-hour movie online in full-screen last night and noticed after about an hour the playback was like in "slight" slow-motion and got a little bit slower.
I was wondering would the lagging problem be solved by installing extra RAM, and if so would I be better off getting either the 8 or 16GB of RAM? Or is it because of the Intel HD graphics 4000?
Thanks.

baller1308 said:

Your Mac Mini should be able to handle it without any issues. You have plenty of ram and the integrated graphics should be able to handle it with ease. Getting more ram won't help. The most likely culprit is your internet bandwidth. How fast is your internet and who is your internet provider?

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Its a 50MB wireless broadband service, practically unlimited monthly download allowance. I got the actual download/upload speeds off speedtest.net: