Maybe use Slack in browser?
While the mobile app is at least responsive on Android, I have problems, too, for example with messages from push notifications that don't show up when opening them.
Subjectively, Slack performed great in the early days but has gotten worse over time.
I run Slack only in Safari. It works significantly better than the desktop app.
/Applications/Slack.app/Contents/Frameworks/Electron Framework.framework/Versions/A/Electron Framework