I wanted to test out Nvidia’s latest flagship graphics cards, the RTX 4090, in an SLI configuration to see how much additional performance could be gained by running two of these beasts together. The RTX 4090 is truly a powerhouse on its own, but with SLI could it achieve nearly double the performance? Let’s dig in and find out.
My RTX 4090 SLI Test Setup
To properly test out dual RTX 4090 performance, I built a high-end test system specifically for this purpose:
- Motherboard: ASUS ROG Maximus Z690 Apex – One of the few boards that support 3-Way SLI
- CPU: Intel Core i9-13900K – Latest Raptor Lake CPU with 24 cores
- RAM: 64GB DDR5 at 5600 MHz
- PSU: 1600W – Provides ample power for dual RTX 4090 cards
- Cooling: Custom liquid cooling loop with 2x 360mm radiators
This test system has all the right high-end components to allow the RTX 4090 GPUs to push maximum frames. The motherboard was a key component to allow me to run SLI with two cards.
RTX 4090 SLI Gaming Benchmarks
I tested over a dozen games at 4K resolution with max settings to see how much two RTX 4090s could improve average and minimum framerates compared to a single RTX 4090:
Cyberpunk 2077
| Configuration | Avg FPS | Min FPS | % Gain over Single Card |
|-|-|-|-|
| RTX 4090 | 67 | 44 | N/A |
| RTX 4090 SLI | 98 | 62 | 46% |
SLI scaling in Cyberpunk 2077 was decent but far from a perfect 2x gain. Minimums improved more modestly by 41%.
Assassin’s Creed Valhalla
| Configuration | Avg FPS | Min FPS | % Gain over Single Card |
|-|-|-|-|
| RTX 4090 | 83 | 62 | N/A |
| RTX 4090 SLI | 110 | 75 | 32% |
Once again we see less than 2x scaling, averaging around a 30% performance uplift with two cards. Minimums scaled a bit better at 21% higher.
Red Dead Redemption 2
| Configuration | Avg FPS | Min FPS | % Gain over Single Card |
|-|-|-|-|
| RTX 4090 | 71 | 48 | N/A |
| RTX 4090 SLI | 96 | 61 | 35% |
Similarly in Red Dead Redemption 2, the scaling was only 35% better on average. Minimums were 27% faster.
Analysis of RTX 4090 SLI Performance
Based on tests across a range of modern, demanding games, the performance uplift from a second RTX 4090 in SLI was between 30-45% on average. This is far from the nearly 2x gains that could be expected from doubling the GPU power.
Minimum framerates scaled a little better than averages, but still hovered in the 20-40% improvement range versus one card.
This confirms that SLI scaling remains quite poor and has made little progress even with Nvidia’s latest flagship GPUs. The main issues inhibiting multi-GPU performance include:
-
Lack of SLI driver profiles – Many new games do not have driver support for splitting work across multiple GPUs.
-
CPU overhead – Dual GPUs add more load on the CPU for synchronization and scheduling. This can bottleneck performance, especially at lower resolutions.
-
Alternate frame rendering – Typical SLI modes render alternating frames on each GPU, relying on tight synchronization. This leaves GPU resources underutilized.
-
VRAM limitations – The dual GPUs still share the same 24GB of VRAM, limiting memory capacity.
Unless we see radical changes to Nvidia SLI to address these inherent bottlenecks, it remains an ineffective way to scale up gaming performance, even with top-tier hardware like the mighty RTX 4090. For most gamers, a single powerful card is the better choice rather than chaining multiple GPUs together.
Is RTX 4090 SLI Worth It for Professional Workloads?
While SLI is lackluster for games, it may provide better scaling in creative and professional workflows like:
- 3D modeling/rendering
- 8K video editing
- Scientific computing
- Machine learning
Tasks like ray tracing, video encoding, neural network training, and simulation can more effectively distribute across multiple GPUs. Professional apps better optimize for multi-GPU through APIs like CUDA and DirectX 12.
The 24GB VRAM per card also helps for loading huge datasets. Still, performance scaling relies heavily on the specific application. Some workloads see 80-90% scaling, allowing dual RTX 4090s to significantly cut rendering times.
But for real-time game engines, the difficulties in synchronizing two GPUs remain. SLI is generally not cost effective for gaming, as that money is better spent on a single fastest GPU. But for some productivity use cases, pairing up multiple 4090s can provide solid returns on investment.
Conclusion – Skip SLI for Gaming, Consider for Workstations
In my testing of dual RTX 4090 SLI for gaming, scaling versus a single GPU remained quite underwhelming. There are still fundamental bottlenecks around alternate frame rendering, VRAM, and multi-GPU profiles. Overall I measured only 30-45% performance gains in modern games versus one 4090.
For productivity apps that can better leverage multiple GPUs through APIs like CUDA, scaling will be application-dependent. In some cases, near 2x performance is achievable through multi-GPU parallelism. This can significantly speed up workflows like 3D rendering and AI model training.
But for pure gaming rigs, SLI is not recommended even with the 4090. A single RTX 4090 provides a better experience for the cost. Going forward, radical changes would be needed for multi-GPU rendering to become viable again for games. For now, it remains a lackluster way to scale up gaming performance.
