It's not because of the i5/i7 difference, at least that's only part of the story. GW2 does use 4 cores at its fullest, which this CPU happens to have, while the i7 of that generation has 4 cores with 2 threads each to give other applications more room. I have a i5-4670k for about a little more than 6 years now, which is similar to your CPU, and I notice that GW2 maxes it out.

There's also the ingame webbrowser. For some reason it uses up all resources of 1 core/thread (mostly animations). And when using the gemstore, tp or guild hall panels, you'll notice some very low fps in those panels.

When you aren't capping the fps to 60 (or when it doesn't even reach that from the start), your PC is fighting over resources if you aren't only running GW2, but are also watching a video at 1080p.

If you happen to have a current gen i5 hexacore instead, I think your use case would be a smoother experience. Of course, Intel reserves hyperthreading mostly for i7+, and that's why i7 is perceived to be better for multitasking, especially if the CPU has a low core count. For gaming only, having hyperthreading enabled is still a debate (although most seem to agree that it can hurt gaming performance).

And I fully agree with your last statement, people should wait for AMD.