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To clear Slack's cache in the desktop app, open the Help menu. The only way to clear Slack's cache on an iPhone or iPad is to delete the app and reinstall it. You can clear the Slack Android app's cache by heading to your device's Apps menu.Slack has a cache that it uses to store messages you've read recently, so it can load those messages faster in the future. And the space that those messages take up on your device —...