If I'm using a mobile app, chances are I'm on-the-go. I probably have a slow, high-latency, or otherwise unreliable connection. It's possible it comes down to protocol differences that hinder UI responsiveness.

Edit: As a test, I just sent a message from my phone while on a cell connection with good service. Hitting the send button felt unresponsive: it took a bit for the message to appear in the chat history (maybe 100-200ms). That's on a stable 300 Mbps connection mere miles from my homeserver.

For contrast, Telegram doesn't wait to clear my text area. I can queue further messages to send even if my first hasn't gone through. Same for Slack and Discord.