What strategies do you use to ensure that complex 3D animations render efficiently without sacrificing quality?

To ensure complex 3D animations render efficiently without sacrificing quality, focus on these key strategies:

1. Optimize Models

Reduce Polygon Count: Use decimation and LOD (Level of Detail) techniques to simplify models, especially for distant objects.

Use Normal and Bump Maps: Simulate details without increasing poly count.

2. Efficient Texturing

Texture Atlases: Combine multiple textures into one to save memory.

Use Procedural Textures: Generate textures algorithmically to reduce file sizes.

Optimize Texture Resolution: Use lower-res textures for background elements or distant objects.

3. Lighting and Shadows

Bake Lighting: Pre-calculate lighting and shadows for static objects to speed up rendering.

Simplify Shadows: Use shadow maps or softer shadow techniques to reduce computational load.

4. Render Settings Optimization

Render in Passes: Break complex scenes into passes (e.g., diffuse, shadows, reflections) and composite later.

Lower Anti-Aliasing: Use less-intensive anti-aliasing options like FXAA or TAA.

Use Adaptive Sampling: Increase sample rates only where needed.

5. Select the Right Render Engine

GPU vs. CPU Rendering: GPU renderers (e.g., Redshift, Octane) are faster, while CPU renderers (e.g., Arnold, V-Ray) provide higher control.

Use Distributed Rendering: Utilize network or cloud rendering for large projects.

6. Cache Simulations

Precompute Simulations: Cache simulations like fluid, cloth, or particles to avoid recalculating every frame.

7. Post-Processing

Limit Heavy Effects: Apply effects like motion blur or depth of field in post-production to reduce render load.

8. Preview at Lower Resolution

Draft Renders: Use lower resolution for preview renders to speed up iteration before finalizing the high-quality render.

By balancing model optimization, efficient lighting, and render settings, you can ensure fast renders while maintaining high quality.

Hope this helps ! 

There are multiple ways to optimize animated scenes in any 3d Application. Some of them includes:
- Using denoisers in 3D applications
This will ensure that the rendered animations stays clean and has less noise in the final output
- Using appropriate 3d props with optimized meshes
Using the 3d models that contains dense meshes will slow down system and will impact on overall viewport performance. So it's always necessary to limit these models to moderately dense 3d meshes.
- Using Render Layers using EXR file format
Any professional 3D artist will choose to export in EXR file format, because of the control it gives to the animator in post production. I have used this technique in my projects to render highly populated scenes, where there are hundreds of 3d models, simulations and  assets that would not load in GPU VRAM just because of the dense meshes and multiple textures used across the scene.

You can test if your animation is suited to be rendered in a real-time rendering engine like Blender's Eevee or Epic's Unreal engine. If you test your scene in one of these engines and it looks mostly unchanged you can save a lot of rendering time. In the case that you are not able to use a real-time rendering engine and you need to use a regular ray-tracing engine I would recommend rendering just one frame first and adjusting the settings to reduce as much rendering time as possible, even a few extra seconds can represent hours when exporting a full animated sequence with many frames. When using a ray-tracing engine there are many settings that you can tweak to reduce rendering time but I find the most important one being the "sample" count, I recommend reducing the samples to reduce the rendering time, this will result in a noisier animation, however then I recommend to use an external program to "denoise" the animation, in the end this will be a lot faster than increasing the sample count when exporting.

Best regards.

If you are working on a big project, and you have a lot of elements, objects etc you should really take into account:

LOD's. Setting up the level of detail according to the distance saves up.

Textures. Having an 8K map on a leaf 10 meters away isnt the best thing.

If lets say you have a stable camera, and something in your scene is still, you can render region the action, and just render a single image for the remaining, motionless part of the scene.

Diving down, knowing tricks to make fake caustics with GOBOS is the ultimate hack for saving render time.

There's an array of actions that can be taken at every stage or production pipeline, from optimized topology of 3d modeling, through textures resolutions to materials parameters. There are also restoration techniques in post-production.
To be honest though, rendering time is not such a big deal as it used to be when I started in the 3d field. Personal computers got much stronger and software much more efficient. There still are aspects that can take tons of time, like complex volumetric simulations, but personally I can't complain, and it's been years since I last needed to outsource computing power.

Always Render Farm is the best one but it`s expensive, The new generation of GPU calculating is the best way to do it in one PC, Use Unreal engine or other new generation of GPU rendering!