I found two things.
1- the file isn’t closed, and selecting all and join makes a buggy file, so select all but one of the base side surfaces and join, then join that surface to the polysurface and it closes up nicely to a closed polysurface. You can double check that by using a clippingplane and slice it through the model.
(@theoutside can you verify that?)
2- the meshing sure is slow at the given quality. When I used to do plastic part modelling, back in the days,I always used custom mesh settings and this is what I used:
I also found that using “Simple Planes” is the part of the meshing process that takes a lot of time on complex models like this (Yes a part with 3116 surfaces IS considered a complex model).
With it OFF it takes 1 second to process the render mesh, but with it ON it takes 9. That alone is a reason to use custom render mesh settings, as you benefit very little from Simple Planes.
This is what the mesh looks like:
PS! If you JUST want to turn off simple planes then you can leave the quality slider at where you want it and hit “Detailed Controls” and toggle off the simple planes option. That makes the file mesh in 1 second as 3116 surfaces and in 3 seconds as a closed polysurface (So my custom settings are faster).
And IMO the mesh is better. Here are the render meshes extracted (with the extractrendermesh command). Your Rhino files default setting with “Simple Planes” turned off to the left, and my custom settings to the right:
It’s both cleaner, faster,has better meshing of fillets and is half the size (48k vs 101k faces) in cases like this.
BUT BE AWARE!
This is UNITS driven so don’t use these settings on a house, you have to use values that reflect the detail level you want. In your file you have text that is 0.1mm wide, so the meshing has to be tiny. So on larger objects using 10 mm for max edge length and 0.1 as max distance to surface would be much better, and on a house you can use 100 and 1.
So play around with them, or just use the slider and toggle off the “Simple planes” if you want it to be geometry driven and not size driven 