The initial question was how to set the speed on an animation.
Here is an animation with different speeds on some of the layers. GIMP defaults to 100ms if no speed is specified. I've also included a screen shot of the layers.
Note that the speed is also followed by (replace). That means each layer is replaced by the previous one. Saulgoode's anim-settings.scm script is attached. Put it in your scripts folder. You can set the speed and you can choose replace or combine mode. I recommend replace for now.
Refresh your scripts. You will find the settings script in
Filters > Animation > Settings
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