Running iRacing at 240hz is one of the best upgrades you can make to your feel for the car. However, many drivers notice that the moment they turn on a SimHub overlay, the game starts feeling like 60hz again. The smoothness disappears, input lag creeps in, and the FPS counter keeps showing 240. This is a known issue with three documented causes, and all of them are fixable.
Why the FPS Counter Lies
Before covering the causes, it helps to understand why this problem is so disorienting. iRacing overlay stuttering does not show up on a standard FPS counter. The counter reports how many frames your GPU completed, not how evenly those frames arrived at your display. When frame delivery is inconsistent, the game feels sluggish even though the number looks correct.
This is called frame pacing. A frame time graph will reveal the problem clearly, while a standard FPS overlay will not. Therefore, checking frame time rather than frame rate is the better way to confirm the issue. Tools like MSI Afterburner or CapFrameX can show a frame time graph while you drive.
The G-SYNC Frame Sync Conflict
The most common cause of iRacing overlay stuttering is a G-SYNC conflict. When iRacing and SimHub run simultaneously, NVIDIA drivers can attempt to sync against both applications at the same time. At 240hz, the sync fires constantly from two sources. The result is heavy, consistent stutter that presents as input lag and inconsistent frame delivery, even when the FPS counter looks normal.
The fix requires two steps. First, open NVIDIA Control Panel and set G-SYNC to fullscreen only globally. Second, open NVIDIA Profile Inspector, find the iRacing profile, and enable G-SYNC for both fullscreen and windowed modes specifically for that application. This gives iRacing exclusive control of the sync process and removes the conflict with SimHub entirely.
After applying the fix, verify it worked by running a frame time graph in MSI Afterburner or CapFrameX with SimHub active. You should see a consistent, flat line rather than the spiky pattern that indicates bad frame pacing.
GPU Composition Overhead
On some systems, SimHub runs on a different GPU than iRacing. This happens frequently on laptops with both integrated and discrete graphics, and it can also occur on desktop builds depending on which display port the monitor connects to. When both apps run on different GPUs, Windows copies the entire framebuffer between them on every single frame to compose the overlay on screen.
At 240hz, that transfer rate is substantial. iRacing performs correctly in isolation. However, as soon as the overlay activates, the transfer load breaks frame delivery. The stuttering that results feels identical to the G-SYNC conflict, which is why diagnosing correctly saves time.
The fix is to force both iRacing and SimHub to use the same GPU in NVIDIA Control Panel under per-application graphics settings. Set both to the same dedicated card. Also confirm that your monitor connects directly to a port on the dedicated GPU, not to a motherboard port.
SimHub CPU Load
SimHub with multiple overlays and active plugins is CPU-intensive. iRacing is also CPU-intensive. On systems where the processor is already near its limit during a session, SimHub takes what is left, and frame delivery in iRacing suffers as a result.
This cause is less common on modern processors, but it remains a real source of iRacing overlay stuttering on older hardware and in CPU-heavy sessions with large grids. The frame rate number stays the same in this scenario. The frames simply are not arriving at consistent intervals, because the CPU cannot sustain both workloads evenly.
The fix is to open SimHub and disable any plugins you are not actively using during a session. Fewer running plugins means less CPU overhead. Additionally, confirm that iRacing is set to Borderless Windowed mode, which is required for overlays to display and also reduces GPU compositor load compared to some other configurations.
Which Fix to Try First
If your system uses G-SYNC and runs on a single discrete GPU, start with the driver fix. The G-SYNC frame sync conflict is the most common cause by a significant margin, and most drivers resolve the issue entirely at that step.
The GPU composition fix applies to any laptop with integrated and discrete graphics, and to any desktop where the monitor connects to a motherboard output. The SimHub plugin audit takes five minutes and costs nothing, so it is worth doing alongside whichever of the other two fixes applies to your setup.
Running overlays and running iRacing smoothly at high refresh rates are not mutually exclusive. The causes are specific, the fixes are well-documented, and a frame time graph will confirm when you have solved it.
