I just did a quick test to see if you might expect to be able to explode a volume of the size of your text voxels without frame spikes. I created a volume the same size as yours, and rigged up a custom exploder and put an FPS graph on it.
Here are the FPS spikes when the explosion radius is set to 1:
Here is the spike when set to radius is set to 2:
This is running on my desktop I7-7790, so on mobile wither of these scenarios will probably cause quite a large hit in frame time.
Here is the reason why:
When you select the voxel object, you can see the individual objects that make up the volume chunks. The more chunks you have to update in realtime, the bigger the hitch you'll end up having. You can see that for an explosion radius of 2 (and a voxel size of 0.1), we have to update at least six chunks, possibly nine depending on where the explosion hits.
For a radius of 1, we have to update two, four or six chunks depending on where the explosion hits.
For each of those chunks, we have to recalculate the voxel mesh, apply it to the meshrenderer, then also recalculate the colliders (concave). That's quite a lot to do, especially on mobile.
My first piece of advice would be to reduce the number of voxels you're exploding by increasing the voxel size. I resized by test volume to 359544, which is half the size of your main text volume. I increased the voxel size to 0.2 so it is the same physical size, but with half the number of voxels. It looks "chunkier", but the frames hit becomes imperceptible, at least on desktop.
I assume you imported that text either via an image or with the Mesh Scanner; both of those tools have an option to set the voxel size, so try reimporting/scanning with voxel size of 0.2 to begin with.
My second piece of advice you found in the other thread, which is to only explode the volume that your bullet collides with, as opposed to using the out-of-the-box exploder, which scans the entire scene for volumes to explode.
To do this, replace the last two methods of your bullet script with the following: