When you talk about a "rendered animation" I suppose you're talking about the video result in FFMEPG format or else, and not what you see in Rendered or Object mode in your 3D view. In your 3D scene, in Solid mode, if you have a heavy scene with a lot of objects, it won't be able to play 24 images per seconds. If you want to have an idea of what is the real speed you need to go in the Timeline window > Playback menu, and enable Frame Dropping option. It will respect the real speed but at the price of some frame droppings.

When you talk about a "rendered animation" I suppose you're talking about the video result in FFMEPG format or else, and not what you see in Rendered or Object mode in your 3D view. In your 3D scene, in Solid mode, if you have a heavy scene with a lot of objects, it won't be able to play 24 images per seconds. If you want to have an idea of what is the real speed you need to go in the Timeline window > Playback menu, and enable Frame Dropping option. It will respect the real speed but at the price of some frame droppings.