[der_general]

I have Kodi/LibreELEC running on a Raspberry Pi 3B+. I also have a Synology NAS which holds my personal photos and videos. I use Kodi to browse images on the NAS, which are mostly in .jpg and .png -formats. Local Area Network is wired gigabit ethernet.

Videos run smooth but I find the browsing experience of pictures to be annoyingly slow. Images open slowly and it takes quite long to switch to the next image. What can be done to speed up picture viewing? Should I enable or disable thumbnail creation? What about caching? Some settings I could check out?

[Klojum]

As far as I can see...

Your local area is gigabit, but the RPi3B+ 100Mb/s port only does 40-50Mb/s at best (connected to a USB2.0 port)
Internal memory available for photos is about 3-400 MB.
The processing of photos is also a CPU matter, so not the forte of a RPi.
Most photos are compressed, so viewing them takes up extra time and possibly more internal memory.
Creation and caching of thumbs is hampered because of poor write speeds on most SD cards.
Videos are processed in small chunks ('streamed'), photos are read as the whole blob.

What can be done to speed up picture viewing?

An RPi4 for starters: a somewhat faster CPU, more internal memory (upto 4GB), gigabit+USB3.0 connections.
LibreELEC running directly from an attached SSD drive. A bit of overclocking wouldn't hurt as well.

[der_general]

As far as I can see...

Your local area is gigabit, but the RPi3B+ 100Mb/s port only does 40-50Mb/s at best (connected to a USB2.0 port)
Internal memory available for photos is about 3-400 MB.
The processing of photos is also a CPU matter, so not the forte of a RPi.
Most photos are compressed, so viewing them takes up extra time and possibly more internal memory.
Creation and caching of thumbs is hampered because of poor write speeds on most SD cards.
Videos are processed in small chunks ('streamed'), photos are read as the whole blob.

What can be done to speed up picture viewing?

An RPi4 for starters: a somewhat faster CPU, more internal memory (upto 4GB), gigabit+USB3.0 connections.
LibreELEC running directly from an attached SSD drive. A bit of overclocking wouldn't hurt as well.  

I quickly skimmed through THIS article and it seems like setting up SSD "booting" is more difficult and time consuming than setting up USB thumb drive booting. Would it be a meaningful improvement to boot from a USB thumb drive (with better read/write speeds than an SD card) compared to booting from an SD card?

[Klojum]

As the article states, USB booting on the RPi4 is not (yet) possible. It is already possible on the RPi3B+.

With the RPi4, you can connect an external USB 3.0 adapter/case (make sure it supports TRIM), and put in a SSD drive.
Then you will still boot off the SD card, but you subsequently fully run off the SSD drive, with theoretically much higher read and write speeds via USB3.

With the RPI3B+, you are still tied to USB2.0 speeds, so not much would change. Write speeds should improve a bit. You could however implement a larger (SSD) drive. USB sticks are short term solutions to me, as they don't have TRIM support. USB sticks are cheaper of course. It's up to you which path to take.

USB booting is no different from SSD booting. You will need to prepare them in the same way.

[der_general]

As the article states, USB booting on the RPi4 is not (yet) possible. It is already possible on the RPi3B+.

With the RPi4, you can connect an external USB 3.0 adapter/case (make sure it supports TRIM), and put in a SSD drive.
Then you will still boot off the SD card, but you subsequently fully run off the SSD drive, with theoretically much higher read and write speeds via USB3.

With the RPI3B+, you are still tied to USB2.0 speeds, so not much would change. Write speeds should improve a bit. You could however implement a larger (SSD) drive. USB sticks are short term solutions to me, as they don't have TRIM support. USB sticks are cheaper of course. It's up to you which path to take.

USB booting is no different from SSD booting. You will need to prepare them in the same way.

Actually I do have an SSD already connected to the RPI through THIS adapter (I use it for DVR purposes) so I guess I could try the SSD booting. Thanks for the input.

Ps. the SSD is formatted as NTFS due to prior uses. Should I reformat it as EXT4 for better suitability for the Linux-based LibreELEC?

[Klojum]

Should I reformat it as EXT4 for better suitability for Linux-based LibreELEC?

Any drive 'permanently' connected to a Linux machine should be EXT4 formatted, but that is my personal opinion.