The Russian military doctrine is quite different from the US one. It places far more importance on artillery and anti-air forces than on air superiority. The Russian army clearly sucks at maneuver warfare and together with the unrealistically optimistic views which were prevalent in the Russian government (read Putin), it explains perfectly well the extremely poor performance in the first months. The performance in the recent months shows results of a more "comfortable" for the Russian army mode of warfare.

Also note that the Russian army was not "fully committed", it was not using conscripts (there was a small scale deployment of conscripts, but after the public scandal they were quickly removed from Ukraine) and did not fully pull forces from all its military districts.

Meanwhile Ukraine was fighting in the total war mode from the first days (they do not pull "recruits" from streets in the broad daylight into military buses just for the fun of it) with huge external support. And having the well trained by the West ideologically charged army backbone with 8 years of practical warfare experience has helped immensely in the first months.