If the red lasts for less than 3 seconds, usually there is another way to tell the mechanic is going to happen:

- Cast name (you can see this by the boss HP bar).
- Animations the boss does (this is rarely used compared to cast names, but is a thing sometimes).
- Environmental clues (this requires panning your camera around the entire arena to see what's going on). This has been increasingly common over the years, and is probably what you are struggling with.
- Patterns (such as one AoE followed by another AoE next to it).

The newest dungeons have a lot of environmental clues and patterns like that, so it's probably what you are struggling with. Ultimately, you just need to look around and try to work out what the "tell" is meant to be rather than always relying on a red indicator.

Generally casts in the game tend to take about 3 seconds to execute and snapshot, even when there's no actual cast bar, even in Savage, so there is likely a tell you're missing.