That's interesting. I'll try to figure out why.
My hunch that you might have been streaming music seems to have been correct, in which case the router would definitely be involved more directly.
I was going to suggest using a better router - as I wrote earlier, a lot depends on what switching protocols might be active, or what priorities the router uses to route packets. Cheap - simple ones might still use contention protocols - or less well implemented ones - which could very easily screw things up.
It is also possible that your pattern of usage has changed over the last few days. If things get worse again, let us know. Similarly, if you think more can be tweaked ....