I imported a bunch of frames as layers and did Filters → Animation → Optimize (for GIF). I got the layers transformed and delay applied to each layer’s name. In my case it is "(100ms)". But I’d wanted this delay to be 50ms, so I could then set specific delay for some frames manually.
You can specify a default delay upon export of your gif.
Since this is a bit more tricky here is what you do, to reset all delays on each layer:
Export your animation as a GIF. In the GIF Export Dialog, set the
Delay between frames where unspecifiedto
50msand export it to
Tick the checkbox below labeled:
Use delay entered above for all frames. This will reset the delay on all layers to
Import your GIF again. All layers should now state the
50msdelay. You can now set the custom delay on specific layers. Export the GIF again by using Export....
Make sure you untick previous settings. This time you don't set anything and export it with the default settings.
Any additional details, are described in the documentation under File Formats.