You're not following i already tested High Sierra as you can see way up top it's only 2MB faster with my AFP test. I need someone to boot into an OLDER osx version 'aka' Tiger - Sierra and compare results with Mojave.
< Is it possible to get better performance in any previous OS? NO, because it's already maxed out.
Why YES it is, because i've tested this out. Try for yourself if you don't believe me. You'll get higher speeds on older osx versions. I mean old not new.
1Gbps network = 125MB/s ABSOLUTELY MAX. And this is theoretical max. As you can see from my screen capture, 113.5MB/s in Mojave. This is the practical max we can achieve via a 1Gbps network. What's the point to try any older OS if THIS IS THE MAX POSSIBLE WE CAN ACHIEVE WITH THIS HARDWARE
In your case, you only get 30MB/s, that's of course still has room to improve, BUT NOT IN MY CASE, IT'S ALREADY MAXED OUT! Can you follow this?
Trust me, I am trying to help you. I did read all your post, and think about what it means, and then tell you what I know.
You told me that you tested "4 types" (which actually means 4 different generations OSX / macOS), and which shows the later the worse. So, you suspect the OS version make the difference.
And what I showed you is Mojave has NO issue on AFP. It can FULLY utilise a 1Gbps network. Going back to Tiger won't make a 1Gbps network become 2Gbps or 10Gbps. So, in my case, please tell me what's the point for this test? If there is any difference, the older OS can only be slower, but not faster. It won't help to solve your problem anyway.
If you believe upgraded from a "dirty" OS causing this issue. Then you don't need 4 hours to test it, just 1 hour should be enough. Get yourself a spare hard drive, install a clean Mojave on it, and test the AFP connection.
If still slow, then nothing about clean or dirty, but something is wrong.
I assume you boot the same Mac with same hardware but just different OS, then achieve different results. Therefore, must not the network card / cable / switch / router etc.
And since I already show you Mojave can do 110MB/sec locally
(I believe this is what you want to achieve from your original post). Therefore, the OS itself is not the issue (in fact, High Sierra can also do 110MB/s).
Then the remaining possible suspects are the settings, AND the other end of the network. Since you said your setting similar to my setup. So, this should not be the issue as well.
In my case, the other end is a QNAP NAS, which may have very good support on AFP. In your case, I don't know. If the other end is another Mac that running Tiger, may be the Mac Pro side running any newer OS really can causing the performance difference (due to poor support from the other side?).