Christopher, any changes?

I reset the preferences, but no change in AI load times. With or without the panels, including the 3D and Effects panel, the time to open a new document takes about 50 seconds. In addition to a cold start of Illustrator, the total time can take up to two minutes. Each time I launch Illustrator, the Progress dialog, 'Rending '3D and Materials' effect appears. I also see the 'drop shadow' progress appear, but only for a brief moment.

When I finally get to the artboard view, I see on the right the Properties/Layers/Libraries. The Libraries tab has hundreds of files. I wonder if that could be a cause, but I don't see how it would relate to the the 3D Effects taking so long to load.

BTW, my Preferences folder for AI is massive: 40gb. I looked into the Recovery folder in particular and it's about that size. It's been backed up. I might find an old backup of it and see if restoring it has any effect.

I suspect I need to reinstall AI at this point. So I'll need to figure out how to back up my User Libraries and load them back in again.

Much thanks Igor for providing assistance and suggestions. I have already taken much of your time, but it's been very appreciated.

I think it’s time to backup Preferences folder (for the am to study later), https://helpx.adobe.com/illustrator/using/setting-preferences.html
The reset Preferences and check if this becomes any better.
40 seconds is a nightmare...

The first time I open Illustrator after I have loaded the MacOS desktop, or just from a cold start of Illustrator, it takes about 50 seconds to get to the 'welcome' screen. If I didn't have the 3D panel open from a previous time, each file takes the same time (under one second). However, if I repeat the same as above, but had activated the 3D Panel from the last session of Illustrator, it takes an additional 35 to 40 seconds to open the first file. If I close the file and open it again, or, the other file, it loads in under a sec.
Now when I launch a new file from the welcome screen, it takes about 30-39 sec (AI already running) to sometimes well over a minute (from a cold start of AI), to get to the working environment.
Opening the files you provided does not show any status bars, only the spinning pinwheel. But when starting a new document, I get the dialog 'Rendering '3D and Materials' effect'.

What a weird thing.
I received a blank document from Christopher with 2 A4 artboards in it.
Whenever I launch it, with or without 3D and Materials panel opened, without or without unused extras, with or without PDF-compatibility, it takes Ai 13 to open it and 13 seconds to close it.
The same document created based on my own Print document profile (unchanged form the stock one), took 5 seconds the first time, and only 2 seconds each time I tested it later.

So these documents (attached to this comment) are virtually identical (except for maybe positioning on canvas), but are different when the code within is compared...
Christopher, can you try to open and close each one of these a couple of times and measure the time this takes at your side. please? Both have the PDF-compatibility disabled and all the styles-brushes-swatches removed, to make it as lightweight as possible. Does having 3D panel opened change anything? One other difference is that I didn’t get any progress bars at all, like you did. Will you get one with these files?

What more occurs when I start a new A4 document, which my default I set to 300ppi, the dialog, Rendering '3D and Materials' effect
appears for at least 20 seconds. I also see a brief dialog for 'warping' and 'drop shadow'. None of that should be loading with a blank document, right? I made a screen capture video of the process. I couldn't see the warping or drop shadow status in the dialog in this recording but it has appeared.

For me it’s not that clear... the very first document I create takes ~7 seconds. Then, when I try to create another one, it’s 2 seconds — with or without the panel opened.

Can this issue be addressed with some sort of priority? I am still having this issue. Even when starting a New, blank document, if the 3D Effects panel was active from the last open session of Illustrator, the time it takes to just get to the artboard view takes upwards of 40 seconds.