Clunky, outdated, unoptimised, and broken UI. Let's compare this to WhatsApp, another Facebook product. WhatsApp immediately sends u notifs. WhatsApp doesn't stutter or lag when sending attachments, typing messages, or scrolling through chat. WhatsApp has tonnes of useful messages features like a pausable voice record, a high bitrate call feature, the ability to send attachments, share music, etc. Messenger has none of these features, it's been abandoned by the developers who clearly don't care.