There are two things that made it 5+ hours to cut (the way I did it, anyway):
First, the edges of the letters are not orthogonal to the surface, they have some slope to them, so you can't cut them with a square end mill and a deep DOC, you have to use a ball mill and a tiny DOC on the watermark tool path.
Second, since it is going to be a mold, there needs to be extremely high quality.
Combine these two, and you end up with a very small step-over and DOC for the finishing path. Since there are lots of tiny pockets that need to be milled out in the letters, and VCarve doesn't support real rest toolpathing, you have a trade-off between using a large-ish area clearance tool (0.25) and not being able to mill out the letters at all, and then taking forever on the finishing pass with a tiny end mill, or using a much smaller area clearance tool (0.125 or 0.063) and spending much more time on roughing.
I think VCarve Pro and Aspire have a much better way to do true rest machining, so you could do multiple roughing paths with progressively smaller bits, but my version doesn't support that.
If anyone else has suggestions for overcoming these challenges, I'd love to learn them!