There are a number of issues that can lead to stuttering or freezing, especially in VR.

Adjust Number of Max Prerendered Frames

Limiting the number of prerendered frames can help reduce perceived input latency, but can also introduce issues, especially stutters.
1. Open the iRacing graphics options

2. Set the Max Prerendered Frames value to 1 and test. If you have an SLI setup you may need to set this value to match the number of graphics cards in use.

Disable Windows Xbox Game Bar Features

Occasionally issues can be caused by the Xbox Game Bar that is integrated into Windows. Aspects of this can be disabled to remove the risk of interference.
1. Open the Windows Start menu and click the gear icon

2. Select Gaming

3. In the Captures section, ensure both toggles are disabled. One is background recording and the other is audio recording.

4. In the Game Mode section, disable Game Mode.

5. If the stutter persists, the entire system can be disabled in the Game Bar section.

Lower the Number of Cars Received and Rendered

The amount of processing required to render a scene is highly dependent on the number of cars. Lowering this can be an easy fix to graphical issues.
1. Open the iRacing graphics options.

2. Lower the number of Max Cars in the text box, and use the dropdowns to lower the number of cars rendered. Lowering the number of pits may help as well.

Disable Asynchronous Reprojection (SteamVR) or Asynchronous SpaceWarp (Oculus)

This can sometimes have different names, but the functionality remains the same: a single frame is repurposed and used a second time after some slight motion is applied. This can feel like a stutter because it is functionally similar to one.
1. For SteamVR, click on the display mirror and then hit Shift + A to disable Asynchronous Reprojection until SteamVR is restarted

For Oculus, click on the display mirror and then hit Ctrl + NumPad_1 to disable Asynchronous SpaceWarp until Oculus is restarted.