Jamieson, Fausset and Brown on Isaiah 66:8

8. earth--rather, to suit the parallelism, "is a country (put for the people in it) brought forth in one day?" [Lowth]. In English Version it means, The earth brings forth its productions gradually, not in one day (Mr 4:28).

at once--In this case, contrary to the usual growth of the nations by degrees, Israel starts into maturity at once.

for--rather, "is a nation born at once, that Zion has, so soon as she travailed, brought forth?" [Maurer].