Slack is an Electron app, taking about 400-500 Mb of memory and it won't be good no matter if it is secure or not. Also, its UI is over-complicated and looks like it was made by a programmer instead of a designer (by the way, Telegram is the opposite: it is written with Qt and has nicer UI).
If you use Telegram a lot, you should check out Telefuel.com. We're building Telefuel for power users and teams, and recently rolled out a feature to filter for unread messages. Might be useful for you!
