Slow response before Express advances from splash screen to actual menu of the program

 I just tried the same with another desktop PC and with freshly installed GE. The results were also  the same with the delay from the splash screen to the active GE menu matching the 18 minutes obtained with the first desktop PC. Seems I am the only one dealing with this kind of behavior of GE...

As a test, and I'm not familiar with the Fenix watch, can you make copies of these suspect .mp3 files on some other storage media, then delete them from the Fenix 6S Pro and then retest?  If the response of GE is significantly different, that at least would support the mp3 files being the issue.

Yes, that is exactly the thing I am going to do next. Since the release of the disastrous GE 7.3.0 which unintentionally deleted all .FIT files from your watch, I have been keeping regularly updated copies of my watch contents in my PC - just in case anything like this would happen again - and among those are also all mp3 files from the watch. It is just a matter of deleting some, if not all, of those mp3 files from the watch and see the impact to the startup time of GE.

If it turns out, that reducing the number of mp3 files also reduces the startup time of GE, is Garmin then going to release a corrective update to GE, or is Garmin going to tell that this is a feature of Windows and not a bug of GE. That remans to be seen...

Something to try first and based on an experience I had similar to your delay between the splash screen and the actual opening of GE.  Several years ago I had a similar experience; posted in various forums and never got resolution.  Sometimes the time was very long, 5 to 10 minutes, but GE would show in my tray.  Things I tried included uninstalling, cleaning the registry, and reinstalling; the problem continued through several GE updates.  I finally decided, just on a whim, to run the Windows Compatibility Troubleshooter and the problem was immediately resolved.  Startup times now have never failed to be more than about 10 seconds and I do not run GE in the background.  To run the troubleshooter, if you have a desktop icon, right click on it and in the drop down select "Troubleshoot compatibility".  Let Windows do its thing, in my case the troubleshooter chose Windows 8 as the best match.

Actually., before starting this thread, I already experimented with all different settings for GE including compatibility with previous Windows releases, running it as administrator etc. None of those made any difference to the staftup time. Next I am gonna reduce the number of mp3 files in the watch to half of the original number and report the results here.

If you open GE without the watch being attched to your PC, does it (GE) open in a normal amount of time?  Normal in my case is about 10 seconds.

And another long shot.  In the search box (lower left corner) type in AutoPlay.  At the resulting dialog box, in the "Removable Drive", use the drop down menu to verify that "Take no action" is the choice.  One of the choices is "Open folder to view files (File Explorer)".  I'm seeing some indications that if that choice is enabled, then File Explorer will attempt to "catalog" (for lack of a better term) the files on the device.

You are not the only one with this issue.....

Thank you for your interest in this problem mine and your kind attempts to solve it!

If I open GE without my watch being attached to PC, GE opens in less than 10 seconds.

The AutoPlay for all media and devices (in Windows 11 the path is Settings => Bluetooth & Devices => AutoPlay) has been turned off so the AutoPlay default actions should no even be applied. Just for in case, "Take no action" has been anyway selected as the default AutoPlay action for both removable drives and memory cards.

What I have not reported so far, is the fact, that while the splash screen of GE is being displayed, the Windows Explorer also freezes and refuses to show content of any drive or directory. This has happened from the beginning and is not related to any possible corrective actions listed in this thread. After about 5 minutes since he splash screen was initially displayed, Windows Explorer comes alive again. This points to the direction, that Windows or GE is somehow indexing the contents of the watch and the number of mp3 files makes this last for soooo long. But if this takes only 5 minutes, what delays the display of GE menu screen the extra 13 minutes?

I also finally experimented with cutting the amount of mp3 files in watch to half from the original of 2300 pcs., and that reduced the GE booting time from the original 18 minutes to roughly 4 minutes. I think this would me in line with the indexing theory of the previous chapter. If this is something that also other users (among them at least atlas_cached) are suffering, Garmin should not expect the users to solve it but take the necessary corrective actions itself.

Have a look at this Wikipedia article on Media Transfer Protocol:

https://en.wikipedia.org/wiki/Media_Transfer_Protocol

When you look at the performance limitations of MTP you can see how multiple applications (Garmin Express, Windows Explorer, Windows Defender, etc) are trying to access the device at the same time performace can fail dramatically.

Great path on those diagnostics!  Now, maybe something that can prevent Explorer from searching that watch.  With the watch connected, type "Indexing Options" into the Windows search box (lower left).   That should yield a screen with a list of attached devices.  Hopefully the watch will be listed.  Click on it and explore the options at the bottom like Modify, Advanced, and Pause.  It is possible to exclude a device from indexing, or even folders on that device.