Set the number of concurrent compiler threads for Unity3D

SetThreadCount.cs

using System;
using System.Reflection;
using UnityEditor;
using UnityEngine;

public static class SetThreadCount
{
    [InitializeOnLoadMethod]
    static void Init()
    {
        // change this to test out different configurations
        // in my tests there were pretty negligible performance differences between various options (*2, *4, *0.5)
        // an easy way to test compilation performance is with https://github.com/needle-tools/compilation-visualizer
        int threadCount = SystemInfo.processorCount;
        
        if (threadCount != SystemInfo.processorCount) {
            // EXPERIMENT: set thread count for compilation "UnityEditor.Scripting.ScriptCompilation.EditorCompilationInterface"
            var eci = Type.GetType(
                "UnityEditor.Scripting.ScriptCompilation.EditorCompilationInterface, UnityEditor, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null");
            if (eci != null) {
                var instanceProp = eci.GetProperty("Instance", (BindingFlags) (-1));
                if (instanceProp != null) {
                    var instance = instanceProp.GetValue(null);
                    var setComp = instance.GetType().GetMethod("SetMaxConcurrentCompilers", (BindingFlags) (-1));
                    setComp?.Invoke(instance, new object[] { threadCount });
                }
            }
        }        
    }
}