Defence in depth based on heavy deployment of mines will stop anything making a rapid advance. The thing Ukraine is short of is equipment to cut paths through the minefields; there are reports that the Russians have been laying stacks of anti-tank mines, 4 or 5 on top of each other, which means that where the Ukrainian army does deploy mine clearing machinery, designed to resist the explosion of single mines, the force is multiplied and thereby knocks the mine sweeper out of action.
Cheap, effective defence, which mitigates the lack of training of the mobilised Russian conscripts, and plays to the strengths of Russia's most effective weaponry, it's artillery.