DisplayPort vs HDMI: Impact on High-Refresh Rate Benchmarks

This article explores how the bandwidth differences between DisplayPort and HDMI connections directly impact high-refresh-rate computer hardware benchmarks. By examining raw data transfer limits, the role of Display Stream Compression (DSC), and the relationship between GPU rendering and display output, this guide clarifies how your choice of cable can influence performance metrics, system latency tests, and overall benchmarking accuracy.

Raw Bandwidth Specifications

To understand how cables affect benchmarking, you must first look at the maximum data rate of the most common modern standards:

  • DisplayPort 1.4: 32.4 Gbps (25.92 Gbps effective data rate)
  • HDMI 2.1: 48 Gbps (42.6 Gbps effective data rate)
  • DisplayPort 2.1 (UHBR 20): 80 Gbps (77.37 Gbps effective data rate)

Bandwidth determines the maximum resolution, refresh rate, and color depth a cable can carry without compression. In high-refresh tests (such as 1440p at 360Hz or 4K at 240Hz), hitting the bandwidth ceiling of a cable changes how the graphics card processes and sends the signal to the monitor.

Raw FPS Benchmarks vs. Display Output

In standard, uncapped framerate benchmarks, the GPU renders frames as fast as possible into its frame buffer, regardless of what the monitor can actually display. If a GPU renders 500 frames per second (FPS), it will report 500 FPS in the benchmark software, even if you are using an older HDMI cable limited to 60Hz.

However, cable bandwidth will skew raw FPS benchmarks if the tester fails to disable vertical synchronization (V-Sync) or Variable Refresh Rate (G-Sync/FreeSync) technologies. If these sync technologies are active, the benchmark's maximum FPS will cap out at the maximum refresh rate the cable's bandwidth can support. For example, testing a GPU at 4K on a native HDMI 2.0 connection will cap a V-Sync-enabled benchmark at 60 FPS, whereas DisplayPort 1.4 or HDMI 2.1 would allow it to scale to 120 FPS or higher.

The Impact of Display Stream Compression (DSC)

When a high-refresh benchmark exceeds the native bandwidth of DisplayPort 1.4 or HDMI 2.1, the GPU utilizes Display Stream Compression (DSC) to achieve the target refresh rate. Because HDMI 2.1 has a higher bandwidth (48 Gbps) than DisplayPort 1.4 (32.4 Gbps), DisplayPort 1.4 relies on DSC much sooner during high-refresh testing.

While DSC is "visually lossless," it requires real-time processing from the GPU's display engine. In synthetic benchmarks, this rarely reduces raw framerate. However, in extreme high-refresh scenarios, DSC can monopolize internal GPU display heads, subtly altering power consumption metrics and GPU clock stability, which can cause minor variances in benchmark scores between DP 1.4 and HDMI 2.1.

Click-to-Photon and System Latency Benchmarks

Hardware benchmarking is not limited to raw framerates; it also includes system latency testing using tools like Nvidia's LDAT (Latency and Display Analysis Tool) or FrameView. In these tests, cable bandwidth is critical.

Click-to-photon latency measures the time it takes for a mouse click to result in a pixel changing on the screen. Higher refresh rates directly reduce display latency because the screen updates more frequently.

If a reviewer benchmarks a system at 1440p using a lower-bandwidth HDMI 2.0 port (which caps the monitor at 144Hz) instead of the monitor’s DisplayPort 1.4 port (which allows 240Hz or 360Hz), the resulting latency benchmark will be significantly worse. The hardware is capable of faster frame delivery, but the bandwidth bottleneck of the HDMI port artificially inflates the system latency metrics.

Frame Pacing and 1% Low Metrics

When benchmarking 1% and 0.1% lows (which measure frame pacing and stutters), the interaction between the GPU's display engine and the cable bandwidth matters.

If a cable is operating at the absolute limit of its bandwidth capacity, or if it is of poor quality and struggles to maintain the required signal integrity for high-refresh rates, it can induce micro-stutters or momentary black screens. While the GPU's internal rendering pipeline might be smooth, external hardware capture cards used in professional benchmarking setups will record these bandwidth-induced dropouts as severe frame time spikes, ruining the 1% low data.

Summary of Best Practices for Accurate Benchmarking

To ensure cable bandwidth does not invalidate high-refresh hardware benchmarks, testers must follow specific protocols:

  1. Disable Sync Technologies: Always turn off V-Sync, G-Sync, and FreeSync when measuring raw GPU rendering performance so the cable's refresh rate limit does not cap the framerate.
  2. Match the Cable to the Generation: When testing latency at 4K 240Hz, prefer DisplayPort 2.1 over HDMI 2.1, and HDMI 2.1 over DisplayPort 1.4, to minimize or eliminate the reliance on DSC.
  3. Verify Display Settings: Always confirm in the operating system that the monitor is running at its maximum rated refresh rate before initiating latency or frame-pacing tests.