All right, if You are sure about it.
There is plenty of great advice mentioned here already. You can use all of those.
I will try to make it simple and short, what You can do quickly and dirty, right now, to make it work.
Keep in mind that it’s only a small fraction of what could and must be done.
Be sure to read Godot documentation, if You are not sure about any of this.
First, as already mentioned, consider starting with measuring what exactly is slow and how slow it is.
A simple way to do it is print(Time.get_ticks_msec()) before and after the code You want to measure.
Reduce the size of Your chunk to 64 or 32. Less work, less time required.
Limit chunk updates to N per frame. Less work per frame, more FPS.
Use Array instead of Dictionary, to store Your data. Godot Array is slow. Dictionary is even slower.
Do not use Godot Node system to traverse Your data, e.g. get_parent, etc. It’s not designed for such heavy use.
Okay to call a few times, not okay to call for 100k voxels.
Avoid frequent Object.new calls. Each requires Godot to bother OS for object allocation. Which is very slow and taboo in hot, rendering paths.
Move all block gen stuff to chunk script, to avoid Object.new.
Do as less operations per voxel, as possible. Precalc and reuse stuff, instead of calc each time on the fly.
It’s ugly and GDScript fault, but reduce amount of function calls. It’s a death by thousand cuts in case of 100k voxels.
Instead of Array.append/append_array, Array.resize first and access elements by index with [] operator.
Use typed Array[T] instead of Array, if possible. Use PackedArray, instead of Array[T], if possible.
Avoid any additional vars, while processing each voxel, instead, insert data directly to Your chunk data struct.
Sooner or later You will need to implement greedy voxel meshing.
You can use run-length encoding to skip empty voxels and optimize Your data storage.
Those are more advanced topics.
Take a look at this voxels implementation in GDScript.
It is slow, but maybe would be enough for Your use case.
At least You can use it as an example.
https://github.com/ClarkThyLord/Voxel-Core
Wish You all the best with Your project and insane resolve, to make it to the end)