Why is Discord getting slower and slower?

Discord consumes a lot of resources because it runs on the web and has a lot happening simultaneously - audio, video, message synchronization, notifications, friends' online status. The more servers you join, the more data it keeps in memory. If you have 50 servers open, the app is constantly loading information from all of them, even if you're not using most of them. Add extensions on top of that, heavy bots running on the servers, and the interface rendering constantly.

Another factor is that your machine might be getting slow in general - RAM full of other stuff, processor busy. Discord will suffer along with it if your phone or computer is lagging. Try closing other browser tabs if you use the web version, or clear the app's cache and data. Sometimes uninstalling and reinstalling helps because the version gets corrupted.

If the problem is just on certain servers (lots of people, many active bots), then it's a server issue. But if it's slow everywhere, try lowering the audio/video quality in settings, disabling animations, or removing some servers you never use anyway. It makes a big difference.