The idea is not to take lots of territory, it is a war of attrition. Grind down and decimate the troops of the opponent.
Russians have been successful at that. Creating cauldrons and leaving one side open so that the opponent keeps sending in troops to get shot down.
If they were all about taking it fast, they would have done that.
Again, that is so 18th century thinking. Do you think we retook Kuwait, or eventually took Iraq and Afghanistan by fighting a war of attrition? (I am discounting the eventual loss of both due to stupid politicians stepping in after the battle was over).
The west is teaching Ukraine how to use the weapons they are providing (i.e. the tanks and more that have not seen combat in Ukraine yet). They will be instructing them in MODERN effective combat techniques.
It is stupid to fight wars of attrition and even Putin knows that. He isn't doing that because he is holding back. He is doing it because his army is a paper one, decimated by decades of neglect and corruption within, and he has no choice.