Zombie code (e.g. unreferenced code) should impact compile time marginally since it will be the first to be optimized away. The most important reason to remove is what I refer to as "Write code for people, not for computers". Readability, being able to follow/understanding the code is very important. Header discipline has often a great influence on compile time, certainly when those headers contain implementation.
Glad you were able to do some massive refactoring. Maintaining a good code base is often two steps forward, one step back.
You got me interested to take a look at it. (maybe involved if in need of support)
Absolutely! Join us on Slack in #development. There is lots of activity these days and we need all the help we can get.