@Grumbles I've found that any images over 1000 pixels either vertical or horizontal either won't load, or are slow to load. The sweet spot for HD resolution and speed seems to be in 750-850 range. Don't use 300 dpi, drop it to 144 or 72 dpi as well.
I've done this to all of my images and I'm happy with the look and load speed on a 55" screen.
One last one .. make sure the size of the file is not over 900k. Reduce it to around 600-500k as long as it doesn't screw-up the quality.
I'm running a Pi3b+, a highly modified version of Eudora with custom system pics and logos if you need to know