If you look closer at the maps that Weeb Union operates with (the one where most of Ukrainian fortifications are mapped out) you'll notice that Russia's speed of advance increased once they have passed that major fortified lines (the one that took around a decade to construct).
It does not appear that the Ukrainian government anticipated the Russian army getting through this major defensive line. Which is why they are resorting to such desperate measures as trying to invade Russia themselves. Which so far only served to extend the overall length of the front lines, which certainly works more against the interest of the Ukrainian army than the interest of the Russia army.
