Best way to cache/preload images and other assets for later use?

[ivan]

OP is specifically asking about loading a bunch of images at the start of the program, not about loading in the background. Frame rate is irrelevant.

True, as long as there is no input and nothing moving on the screen (like I said, a static splash screen).
In that case, you don't need threads or coroutines at all.

[Nixola]

You still need coroutines if you want to be sure the OS knows the program is still responding.

[grump]

In that case, you don't need threads or coroutines at all.

My OS shows a "Program is unresponsive. Force quit?" dialog if the program is in a busy loop for a while and the window does not process its message queue. I think Windows does this too.