Minimal internet speed for multiplayer?

I've been playing with a friend and lately she's run into a 'slow-motion' issue. She sent me a video of her screen and sure enough the game was running in literal slow motion. She said it doesn't happen at all if she plays offline in single player, but whenever we play multiplayer, it starts to happen again. But it also isn't constant. She says it comes and goes sporadically. One moment it runs fine, the next she's chugging around in slow-mo.

I had her run an internet speed test at...

Minimal internet speed for multiplayer?