Changing start frame of simulation changes look A LOT - why?

I’m building an explosion scene - the scene is 494 frames long and my first explosion starts around frame 100. It is based on the large explosion preset so there are keyframes on emitter temperature, emitter activity and density reference. In order to create a second explosion I copied the embergen file, selected all the keyframes and moved them down the timeline to around frame 300.

I have two questions - when I run the second explosion it starts where I would expect but looks a lot different than the first, different shapes and expansion. Given that the keyframes are spaced apart exactly like the first one I would expect the second to look like the first one, only later. Can anyone explain why they are different?

Also when I preview it in embergen I have to wait while the sim runs from frame 0 to the start of the explosion before I see anything. Given that there is no emitter activity until around frame 300 is there any way I can start running it later, say frame 250, and avoid the long wait until I see what it looks like?

Many thanks in advance for any info . . .