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:
Any additional details, are described in the documentation under File Formats.