Jamieson, Fausset and Brown on Isaiah 28:4

4. Rather, "the fading flower, their glorious beauty (Is 28:1), which is on the head of the fat (fertile) valley, shall be as the early fig" [G. V. Smith]. Figs usually ripened in August; but earlier ones (Hebrew bikkurah, Spanish bokkore) in June, and were regarded as a delicacy (Jr 24:2; Ho 9:10; Mi 7:1).

while it is yet--that is, immediately, without delay; describing the eagerness of the Assyrian Shalmaneser, not merely to conquer, but to destroy utterly Samaria; whereas other conquered cities were often spared.