Phew! Challenges work fine today! Though I didn't get a reply for support email I sent yesterday, so I don't know if it was something they did or that was a glitch that resolved itself on its own.
Chrome (86.0.4240.75 (Official Build) (64-bit) is still unplayable for me, FPS seems to be lower than 10
Edge (44.19041.423.0) works OK, feels smooth, but I think there are occasional hitches
Firefox (81.0.2 (64-bit) feels best to me at the moment, though it's so close to Edge's performance that this difference could be just in my head.
so I encountered an interesting bug today in the Archers challenge - one of the archers got stuck in the upper left corner of the map, facing away from me. However, when I approached him his firing speed turned out to be so fast I didn't land a single hit on him because he was waster than the sword swinging animation. So I could not attack him at all but I could not leave the area either because of how fast I was getting hit. So I died right there and could just watch
Yeah, they do that when cornered, I think I managed to kill a cornered archer once by clicking as fast as I could and eventually got enough hits through, but generally I'd say approaching them is useless - you are either running too low on time anyway and if an archer managed to walk so far you've already lost; or you have extra time and you can just leave that archer be and focus on killing the following archers as soon as they come.
Fun fact: you can corner an archer if you stand in the gate and attack just as they spawn.