Find out what assemblies are being built and how long each takes.

Unity Assembly Definition Debugger.cs

using System.Collections.Generic;
using System.Diagnostics;
using UnityEditor;
using UnityEditor.Compilation;

[InitializeOnLoad]
public class AsmdefDebug
{
    static Dictionary<string, Stopwatch> s_Assemblies = new Dictionary<string, Stopwatch>();

    static AsmdefDebug()
    {
        UnityEngine.Debug.Log("Setting up");
        CompilationPipeline.assemblyCompilationStarted += CompilationPipelineOnAssemblyCompilationStarted;
        CompilationPipeline.assemblyCompilationFinished += CompilationPipelineOnAssemblyCompilationFinished;
    }

    static void CompilationPipelineOnAssemblyCompilationStarted(string obj)
    {
        var sw = new Stopwatch();
        s_Assemblies[obj] = sw;
        sw.Start();
    }

    static void CompilationPipelineOnAssemblyCompilationFinished(string arg1, CompilerMessage[] arg2)
    {
        var sw = s_Assemblies[arg1];
        sw.Stop();
        UnityEngine.Debug.Log(arg1 + "\nTime:" + sw.Elapsed.ToString());
    }
}