Is it perhaps possible to have an extra preview option ?
A preview where it tries to reuse previously exported image sheets.
I would imagine this could be very handy where people only tweaked some logic settings/events.
Perhaps the image editor could make some sort of a time stamp when the last image is edited/updated in the project, and if the time stamp was unchanged since last preview, reuse the previously exported images.
If that would work, it could significantly reduce preview loading times for those with large amounts of frames/images in their projects.
The only time where my previews became annoyingly long, is when I used lots of decently sized images for animations and all the sprite sheets require exporting each preview run.