I recently bought a (discounted) Dell U3224KBA 6K screen for my home office and it’s a great screen with great productivity options. However, there is an input lag on the screen, which feels like the screen is 55Hz instead of 60Hz and strains my eye-hand coordination when precise aiming. I hope to figure out how to solve this as it’s driving me nuts (should I keep the screen or return it?).

Update: it seems related to my MacBook’s screen being ProMotion, but still I feel something’s off.

Observations & non-solutions ¶

Some observations:

1. The screen is on 60Hz. At 30Hz the lag is much worse.
2. It’s on my Macbook Pro (M1 Pro), Windows laptop (i7 11000 series) and Windows Desktop (i5-8400/GeForce 1060 GTX 3GB).
3. It’s not happening on my MacBook or Windows laptop internal screen with the Dell screen also connected.
4. It’s not due to CPU load as it happens with no programs running
5. It happens on native resolution as well as scaled down (e.g. 1080p)
6. It’s also present with my MacBook screen on 60Hz instead of ProMotion – not sure if fully correct.

Non-solutions:

1. Turning on “reduce transparency” in accessibility settings (from reddit (reddit.com))
2. Firmware update to M2T104
3. Setting macOS wallpaper to black color, removing all windows (hypothesis: DSC causes display lag on complex scenes)

Resources:

• Michael Stapelberg review (stapelberg.ch)
• Frame rate exceeding refresh rate (blurbusters.com)

Quantification ¶

I took a video of dragging a window from left to right, and it seems the delay depends on the cable used

1. 4 frames @ iPhone 240 fps slo-mo video with USB-C cable #1 = 16ms = 1/60Hz
2. 2 frames @ iPhone 240 fps slo-mo video with USB-C cable #2 = 8ms = 1/120Hz

Theoretical requirements ¶

6144 * 3456 pixels^2 * 60 Hz * 30 bits/pixel = 38 Gbit/sec 6144 * 3456 pixels^2 * 60 Hz * 24 bits/pixel = 31 Gbit/sec

Computer & screen diagnostics ¶

Hardware:

• MacBook Pro 14-inch 2021 M1 Pro (apple.com). Thunderbolt 4 output: “Up to two external displays with up to 6K resolution at 60Hz at over a billion colors (M1 Pro)”
• Windows Desktop (i5-8400/GeForce 1060 GTX 3GB (nvidia.com)). “Maximum Digital Resolution: 7680x4320@60Hz; Standard Display Connectors: DP 1.43, HDMI 2.0b, Dual Link-DVI”

Cables:

• USB-C cable #1: Nedis USB 3.1 Synch & Charge cable - SC11888
• USB-C cable #2: Thunderbolt 4 FW 3.0 cable
• USB-C cable #3: OWC Thunderbolt 4 cable (galaxus.de)
• HDMI cable #1: No-name high speed cable
• HDMI cable #2: No-name high speed (reportedly 8K) cable

With USB-C cable #1 on MacBook Pro 2021 ¶

Dell display info:

1. Input: Thunderbolt (140W)
2. Resolution: 6144x3456,60Hz 30-bit
3. Stream info: 5.4Gbps DSC

macOS reports:

• Graphics/Displays
  • DELL U3224KBA:
    • Resolution: 6144 x 3456
    • UI Looks like: 3072 x 1728 @ 60.00Hz
    • Mirror: Off
    • Online: Yes
    • Rotation: Supported
• Thunderbolt/USB4
  • Port (Upstream):
    • Status: Device connected
    • Link Status: 0x2
    • Speed: Up to 20Gb/s x1
    • Current Link Width: 0x2
    • Receptacle: 1

With USB-C cable #2 on MacBook Pro 2021 ¶

Dell display info:

1. Input: Thunderbolt (140W)
2. Resolution: 6144x3456,60Hz 30-bit
3. Stream info: 8.1Gbps DSC

macOS reports:

• Graphics/Displays
  • DELL U3224KBA:
    • Resolution: 6144 x 3456
    • UI Looks like: 3072 x 1728 @ 60.00Hz
    • Mirror: Off
    • Online: Yes
    • Rotation: Supported
• Thunderbolt/USB4
  • Port (Upstream):
    • Status: Device connected
    • Link Status: 0x2
    • Speed: Up to 40Gb/s x1
    • Current Link Width: 0x2
    • Receptacle: 1

With HDMI cable #1 on Windows laptop @ 4K resolution ¶

Dell display info:

1. Input: HDMI
2. Resolution: 3840x2160,60Hz 24-bit
3. Stream info: LEGACY

Windows reports: Todo

With USB-C cable on Windows desktop ¶

Todo