You can test this right now by breaking your circle in half vertically and setting the two halves to be different colors, and therefore different engrave jobs. I suspect you’ll end up with a visible seam in the final product though, so ymmv.
My gut says that it’ll take longer to do it this way but I could be wrong. It’ll probably depend a bit on the size of your piece and how “thick” your circular engraved area is. (As in the “walls” of your circle. )
You might be interested in this thread:
https://community.glowforge.com/t/super-manual-engraving/15965
To truly optimize this you’d slice the bottom and top off tangentially to the inner path, setting them up as separate jobs, and therefore not duplicating any vertical engrave distance unless 100% necessary.