How do I slow down Screen Saver shuffle/random picture?

Thank you. It's helpful to know that I'm not alone, even more so that people were genuinely trying to help.

I managed a workaround without editing system files using iPhoto as follows:

1. In iPhoto prefs, uncheck box marked "Copy items to the Photos library". This avoids overburdening your system HD, and as with iTunes, loads pointers rather than copying files.
2. Import "items" i.e. pointers from your carefully curated lifetime collection of images. In my case, this amounted to ~20k images, meaning you'll never get bored. This took approximately half-an-hour on a Mac Mini 2014 vintage i7 16GB.
3. Select all the above "photos" and add them to a new project in iPhoto.
4. The new project is now a "slideshow" with parameter settings for "Duration" available at the RHS. I found the best settings were: Classic theme, slowest possible Dissolve Transition setting, and longest possible Duration for the entire collection. This resulted (for me) in a tolerable duration of ~1 minute for each image. If I chose a "Ken Burns" effect theme, all of these settings were overwritten with the already familiar ~3 secs.
5. One advantage of this way of doing things is that it disables Screensaver while it's running, so that your art images display tastefully while you have dinner or just contemplate.

Thank you dialabrain.

That’s what I expected...

It’s like having a manic art gallery salesman in one’s living room. Flash Renoir.. Monet.. Manet.. flash flash flash.

No secret hidden pref? No screensaver manager app that doesn’t want to colonize your screen with its own cheesy images?

Sigh.