Since i was scrolling a bit through and you have no answer yet, let me leave some hints.
First, it might be wise to share more remarks on your issue. i see some google drive links there, but i will not check them out, out of security reasons. So describe your issue in more detail and somebody with actual knowledge might be able to help you.
I have no knowledge about RPG Maker or your add on. So i will give just some general advice.
In my experience the best you can do with performance issues is to benchmark them and double check what you were measuring. You mentioned that the performance gets better on smaller maps. Did you try something more, like f.e. there is only one tile. Did you disable all events and loading new elements to check the idle time? Are you familiar with certain techniques, like out of view means disabled for rendering. Did you load every tile again and again instead of resource sharing? Since i have no insight in your code, there are many places where things can go wrong. So with a benchmark you get a good idea where your issue is. As i mentioned double check what you were benchmarking, as it can happen, there is something else influencing your benchmark (in my case writing the logs to console during measuring was bad).
Also checkout tutorials how they do stuff, they might give you an idea how to do something different and with a new benchmark you can see if it helps you or not. I found f.e.
at 1:38 he makes remarks on performance. He also mentions later it uses RAM and CPU...well i hope in 2025 it actual uses gpu. I myself had to move from cpu to gpu as there was no way to get more out of my cpu. And it was not for a 3d game, but a 2d one. (to be fair i have no gaming pc ;) )So i can not solve the issue for you, but i can at least give you some general direction. Dont let yourself be discouraged by it. See it as a challenge, that you will beat with enough willpower. Have fun.