Spoon Maven Plugin is extremely slow with generated Protobuffer-Classes

Hi,
I'm trying to incorporate Spoon into my project to modify generated Protobuffer-Classes.
However when running `mvn install` on my project with the spoon-plugin enabled, it freezes when running the `JavaOutputProcessor`.
After trying to reproduce this in a clean test-project I found out that spoon seems to get really slow when analyzing protoc-generated java-classes. So slow that it didn't finish on my main-project when running it for around half an hour.
I created and attached a simple test-project for you guys to reproduce this. It contains 9 proto-messages that the `protobuf-maven-plugin` will generate java-code for. After that the spoon-plugin will run without any processor configured and needs 1.5minutes to finish on my local machine.
Disclaimer: The generated Proto-class is around 3000 lines big. Is this the problem? Is there a way to speed up the process? I just want to remove the "final"-attribute from all "Builder"-classes.

Guide on how to reproduce:
1. download and unpack the [zip-file](https://github.com/INRIA/spoon/files/2905062/SpoonProtoTest.zip)
2. install the protoc-binary to compile proto-files
- for example [here](https://developers.google.com/protocol-buffers/docs/downloads) or via `brew install protobuf`
3. run `mvn clean install` on the project

What I see:
```
[INFO] --- spoon-maven-plugin:3.1:generate (default) @ SpoonProtoTest ---
2019-02-26 11:49:26,788 WARN spoon.Launcher - Spoon is now using the 'no classpath mode' by default. If you want to ensure using Spoon in full classpath mode, please use the new flag: --cpmode fullclasspath.
2019-02-26 11:49:26,789 WARN spoon.Launcher - Spoon now parse by default the comments. Consider using the option --disable-comments if you want the old behaviour.
2019-02-26 11:49:26,794 INFO spoon.Launcher - Spoon version 7.0.0
2019-02-26 11:49:26,794 INFO spoon.Launcher - running Spoon...
2019-02-26 11:49:26,794 INFO spoon.Launcher - start processing...
2019-02-26 11:49:28,433 INFO spoon.Launcher - spoon.support.JavaOutputProcessor
2019-02-26 11:51:07,720 INFO spoon.Launcher - end of processing:
2019-02-26 11:51:07,720 INFO spoon.Launcher - no errors, no warnings
```

Is this a performance-leak or am I just misusing the library?
Thanks :)