Performance Issues

### Environment

* Version of Malimbe (Github release number) (Github branch and commit hash)
v9.4.2

* Version of the Unity software (e.g. Unity 2018.3)
2019.1.1f1

### Steps to reproduce
Open the attached profiler session.

I'm pretty excited to use Fody and Malimbe, specifically for the XML documentation weaver, but I'm seeing some pretty drastic compilation performance issues on what is really a fairly simple project.

Compilation time before Malimbe: 8-10 seconds
Compilation time after Malimbe: 18-20 seconds

Most of the time is spent in the OnCompilationFinished callback. Some of the time seems to be spent traversing files and another good portion is waiting for an unclear Async task. I've attached a Unity profile session so you can look through it yourself.

Link to profiler session: https://drive.google.com/file/d/1s7df-o4q4h4GWwxODncBMvqlFBnKf3TG/view?usp=sharing

The attached profiler session expands to 4Gb, just a heads up.

Additionally, here is my FodyWeavers.xml
```

Error, Warning
^Zinnia
^Assembly-CSharp

```