They aren't trying to take Ukraine, though, and they can progress faster as frontlines are broken through. Pokrovsk, as an example, is currently encircled by Russian forces and will probably be abandoned by Kiev soon, or a large-scale siege will occur.
Whether you're pro-Ukrainian or not, it's important to recognize that Ukraine is steadily losing ground and has far less staying power in a protracted war than Russia does. Russia's advancing slowly and basically forcing a long-term war, which works in their favor.