Why is Redshift rendering slow on multi-GPU servers?

Many users expect that Redshift rendering should be faster with more GPUs. However, some discover the opposite: when rendering on a server with 4-8 GPUs, the process can actually take longer compared to a workstation with fewer cards, or even their personal computer. This blog explains why Redshift rendering slow on multi-GPU servers and how to improve performance.

The main cause is usually a CPU bottleneck.

The reason is: CPU is responsible for preparing and sending data to the GPU. If your project includes heavy assets, complex shaders, or detailed rendering settings, this pre-processing step can take seconds or even minutes. When multiple GPUs are involved, the CPU workload increases, adding to the loading time before rendering begins.

For heavy scenes, the render time per frame is long enough that the additional CPU load is negligible. In these cases, having more GPUs will provide a noticeable speedup.

For light or simple scenes, the CPU load can exceed the GPU rendering time. Therefore, a multi-GPU server may be slower than one with fewer GPUs.

To check which GPUs are rendering, open Task Manager > Performance > GPU, then switch one of the tabs (3D, Copy, Video Decode, or Video Processing) to CUDA.

If you’re paying for a workstation with many GPUs, maximize its power instead of leaving them idle. Open multiple projects or render jobs simultaneously to keep all GPUs active.

For small or simple projects, using fewer GPUs saves time and money. 

We also tested C4D Redshift render times on single vs 4x RTX 4090 servers with light and heavy scenes. The results showed that with light scenes, the 4-card machine rendered 2.7 times faster, while with heavy scenes it sped up 3.2 times. So, for light scenes, it is reasonable to use a machine with fewer cards, but for heavy scenes, the more cards the machine still wins.

See more about our benchmark test video below.

If you have access to multiple machines, split your project and render in parallel. This not only accelerates the process but also ensures no resources are wasted.

The idea that “more GPUs always means faster rendering” isn’t always true. When you encounter Redshift rendering slow on multi-GPU setups, it usually comes down to CPU bottlenecks and scene complexity. By understanding these factors, you can optimize both speed and cost, ensuring Redshift delivers the performance you expect.

If you often encounter errors or take too long when rendering with Redshift on your personal machine, the optimal solution is to use iRender – the best render farm for Redshift. With a powerful GPU system, you can significantly shorten the rendering time and avoid hardware compatibility issues.

iRender supports all popular 3D software and render engines, including Redshift and all software or plugins needed. We give you access to high-performance GPUs, so you can render faster and more efficiently, no need to upgrade your own hardware. All servers are equipped with:

• AMD Ryzen™ Threadripper™ PRO 3955WX (3.9-4.2GHz) or
• AMD Ryzen™ Threadripper™ PRO 5975WX (3.6-4.5GHz)
• (1/2/4/6/8x) – RTX 3090/4090
• 256GB RAM 
• 2TB NVMe SSDs

Our servers are equipped with the latest C4D and Redshift versions. In addition, as an official partner of Maxon, each user gets a completely free combo C4D Redshift license from us. So all you need to do is transfer files to the remote machine, connect to the machine, and render as on your personal PC.

Unlike other services, iRender lets you upload and download files without starting the server, saving you both time and money. We offer a Desktop app for Windows with a full feature set and a simple Drive app for macOS for easy file transfers. Here’s why iRender is a top choice for your rendering needs:

• 100% Software Support: We support all 3D software, their plugins, and render engines.
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• Real-human 24/7 Support: Our professional support agents and technicians guarantee to bring you the best rendering experience.

Let’s see how C4D Redshift performs on our server!

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