Most people don't get a slow warning. The door worked fine yesterday, and today it either won't lift past a few inches, or it drops fast and hard the second the opener releases it - that's a torsion spring that let go, and the opener's motor was never built to lift the door's full weight on its own.
Other signs: the door feels twice as heavy when you lift it by hand with the opener disconnected, there's a visible gap in the coil where the spring's wound above the door, or you hear a loud bang from the garage with no obvious cause. That bang is the spring letting go under tension, not a car door or something falling.
On older torsion setups, especially the single-spring systems common on houses from this era, one spring failing puts the whole load on a shaft and center bearing that were never sized to carry it alone - which is often how a spring failure turns into a bent shaft if the door gets forced open before it's fixed.