Unreal by default merges all the C++ files into one giant file. This improves performance in AAA games, mainly if you have a lot of files too; but for small developers it’s only a pain.

Very discutible to have it enabled by default, and also without any mention of it.

Open the ProjectName.Build.cs file and modify it like this:

using UnrealBuildTool;

public class ProjectName : ModuleRules
	public ProjectName(ReadOnlyTargetRules Target) : base(Target)
        //always create a precompiled header
        MinFilesUsingPrecompiledHeaderOverride = 1;
        //This will turn off the combining of source files
        bFasterWithoutUnity = true;

Unless it will be the final production build ready for Steam or whatever I recommend to always use this trick to have very fast build times