I have a mid 2014 MacBook Pro Retina that I experience a similar issue with sometimes. It's a 2.8GHz Intel core i7 with 16GB RAM. I've also noticed some sites' video's are slow to buffer while others like Youtube or Netflix etc just scream along no problem. Amazon Prime Video is pretty good too but I notice getting in to the program and getting going with a film is just a tad slower than Netflix. Once running it's fine. But yes.....definitely some sites are problematic. So it could be a website thing if all the advice given already does not pan out.....ie you have a fast, current computer with enough memory etc. I wonder if it could also be a cookie thing or spyware/malware getting in there and screwing things up a little? Maybe the security for these types of things is better at some sites than others. Youtube probably has top notch protection.
But then again........I have no idea whatsoever how that stuff all works-hacking and yada yada.
I just wanted to throw something out there to try forget, even just for a few minutes........that I just stepped on the jack plug that was plugged in to my 335 guitar and it broke the face of the guitar at the output jack. The wood is broken. So bummed!! It could have been a lot worse but I'm still assuming it will be a tricky repair and it will never look the same. Oh well.......I guess I'm not playing it for looks. AAAAaaargh!!!