VRay CPU render don't use full cores?

[baoxiaohe_developer]

VRay sdk version: appsdk-win_qt5-nightly-vray_v5.20.24-20221205
OS: Windows Server 2022 standard
In rendering, I found CPU utilization is not always 100%. so, what settings I should change?

my code is simple:

renderer.RenderMode = RenderMode.PRODUCTION;
renderer.SetImprovedDefaultSettings(VRayRenderer.D efaultsPreset.High);
renderer.StartSync();
renderer.WaitForRenderEnd();