Cannot open slideshows in PSE 2021

One thing in common when I load slideshows in PSE 2021 on all my computers is that they all are using the same catalog.  I thought that by making new slideshows directly in PSE 2021, I could account for any errors in converting from 2018 to 2021.  And I got the same loading delay in both the newly created and converted slideshows.  What I did not try yet is creating a whole new catalog.  And that fixed the problem.  Slideshows take the same amount of time to load in both PSE 2018 and now 2021.  Not individual slideshows, but the whole catalog was corrupted in the conversion, affecting any new slideshows created in that catalog as well as old ones.  Any thoughts on why that would be? Don't think catalog size mattered, because size was the same in both 2018 and 2021 versions.

So that establishes definitively that your slow load problem is different from mine.  I thought they might not be the same, because when I watched GPU activity in the Task Monitor window, I saw no activity in the GPU while the show was loading, and it only spiked when loading was finally finished and the show started to play.

So I will have to have two catalogs going forward:  one pre-2021 and the other post-2021.  Not as I wished, but the best option available.  Do you concur?

When your catalog and all audio, pictures and videos are on a 2TB internal disc drive, and you want to buy a new computer with a speedy 1/2 TB internal SSD, and you supply an external 2TB HDD for audio, video and pictures, it requires a very thoughtful and careful work flow process to achieve the desired state of having the Catalog on the internal drive and the other files on the external drive, and the Catalog still being able to connect with all the files, and yet there not being duplicate copies of all your audio, video and picture files, so even 2 TB would not be enough to store two of everything.

The first problem was that, appearently due to some kind of corruption in the PSE backup engine, I could not make a new full backup.  I had an old full backup and a number of increments, but I could not find any documentation on how to restore from increments.  Moreover, some increments might be missing.  Michel Paris and I had a long discussion about this whole problem about a week ago.  Please see this topic "PSE will not backup; says insufficient space".  If you have any suggestion on how to fix this problem, that would be welcome.  So I determined the only way to go forward would be to buy PSE2021 and see if I could make a backup from that.  That worked.

So before I copied any audio, video or picture files to drive E:, I restored my backup to E:.  I did a test backup and restore first, and found that the option to overwrite existing files of the same name in the same folder in the restore process DOES NOT WORK!  Regardless of the setting of this option, the restore process with make a duplicate of any restored files of the same name that are already in the folder.  Note that if you select restore folder structure, then do not specify any folders in the folder path box for the restoration--just enter drive E: and leave the path empty.  Otherwise, the folder path will be duplicated. For example,  you want your pictures and videos to be in the path "Users/[user name]/" and you put that path in the box.  Then after restoration you pictures will actually be in the path "Users/[user name]/Users/[user name]/Pictures".  The catalog will be placed in the E: root directory; you have to copy it to to C:.  So appearently if you want to change a user name in the restore process, like to the Microsoft default user name for users with a Microsoft account, the old name will appear as a subfolder under the new name.  Not what you want!

I decided to leave the catalog in E:.  That way I could conect the entire catalog and files to my traveling laptop and show anything anywhere.  PSE 2021 has the option to backup the catalog only and I could back up the catalog to the cloud.

The next step is to move my files from the old computer to the new computer using Laplink PC Mover Pro.  You can't use PC Mover Express because that doesn't have any option to redirect the audio, video and pictures to the external E: drive; it will only copy them from one C: drive to another C: drive.  You have to do the PC Mover AFTER the Restore, because PC Mover will not restore to a folder a file which is already in that folder.  So if you have a lot of pictures in the Pictures folder that are NOT in the Photoshop catalog, this is the only sequence (as far as I can tell) to get those pictures on the new computer without creating duplicates of pictures already in the catalog, or doing a lot of tedious manual examination and comparison.

But I put that PC Mover process on pause because PSE2021 NO LONGER DOES SLIDESHOWS!  I can't go backward to 2018 because I cannot get a full backup to restore.  I can't use PSE 2021 because the reason I use it is to make Slideshows.  Even if I could get a full backup of 2018, it might be too late to use it because when I contemplated to move to a new computer with both internal and external drives, I took that opportunity to clean up a lot of problems in the folder and file structure.  I deleted duplicate files, moved some files to other folders, renamed folders.  After each and every change, I ran a reconnect in 2021, but not 2018.  If I had to use 2018 now, there would be a lot of reconnecting to do, and I don't think I could find them all easily.

The explanation above about how to move to a new computer might a good posting in another topic.  Please suggest where.

This might be my last posting about Photoshop Elements.  I have been using it since PSE3, and have 70,000 pictures in the catalog.  I have been working all my otherwise leisure hours these last two weeks to get Photoshop Elements working in my new computer.  I got it working, but then Photoshop Elements 2021 stopped working!

. . .  and I just noticed that you have a newer OS build than me.  Are you using a beta version of Windows?

No, that did not make any significant difference, maybe a couple of seconds, but that could be just because these pix were previously loaded into memory on the first run.  

To put some numbers on this effect, I clocked a slideshow with 72 photos and a one minute video loading in about 30 seconds in PSE2018; on the same computer running PSE 2021, this slideshow took about three minutes to load.  On a newer, faster computer this slideshow still took longer to load, about 45 seconds,  than on the older, slower computer runnning PSE 2018.

So all the benefits of my new fast computer have been dissipated by Adobe carelessness?  Did't anyone else besides you and me find this problem?  It cannot be exclusive to me because I observed it on three different computer running PSE 2021, whereas they all perform as before with 2018.

Some more observations. This lag applies to both slideshows created in 2021 and slideshows created in PSE 2018 and converted to 2021. I find they eventually open, but having to wait SIX MINUTES for a small slideshow to open means this feature is essentially useless.  And I find that there is a similar time difference for creating slideshows as for opening them~5-6X times longer.  
And I previously mentioned that the thumbnail files of the first slide in each show are not all loading in the All Media view.

I am only having this problem with PSE 2021; no problems with PSE 2018.  Can't tell with earlier versions since they use a different engine to create slideshows and they all have to be converted before opening in 2021.   But there are 2 slideshows I created in PSE 12 and converted in PSE 2018, then again to 2021, and these two take interminable time to load. 
I have Windows 10 Home OS build 19041.685