I wouldn't dare say Slack has buttery-smooth performance. The problem with Element is all the little "what the f#ck?" bugs that add up.
An emblematic example is where the Android app sometimes glitches out and makes it seem a message you sent was sent by someone else. To be charitable I heard a fix is "on the way," but wow.
The markdown editor can also be glitchy. When I use italics or the triple backticks for code it won't always render in your client. It still feels like a hit and miss (albeit far more rare these days.)
