Don’t blame CoreELEC for the slowness of processing photos. It is a Kodi problem.
And again, an internal NAND installation wouldn’t be noticeable faster than your sd card.
Did you try to disable thumbnail creation in Kodi settings like advised above?
Take a look at “top” (using puTTY) while viewing photos. Your CPU usage is most probably at 100%. Even if your thumbnails were already built, it’s still hammering your CPU, if you don’t disable thumbnail creation.
If you want something more powerful, take a look at the upcoming Odroid N2.
This is also one of the few devices, where “installtointernal” is officially supported (it isn’t on cheap Chinese boxes).