Discord servers as I have experienced them are just big groupchats. I'm not the most familiar with the forum style of internetting, being a bit young to have really gotten any before social media took over, but everything feels much more fast paced on Discord. Even with notifs off there's this kind of demand for your attention - like everyone is talking at you that's hard to shake. For me that's why they feel like so much effort compared to (for example) this comment thread where I can peruse at my leisure.