No doubt it's complicated, but some key factors are

• Availability of ammunition (cluster munitions may help here)
• Destroying artillery before advancing (the daily numbers have been looking good)
• Removing mines before advancing
• Disabling logistics (e.g. strikes on bridges, warehouses)
• Waiting for a lucky opportunity

It doesn't cost them much to do things slowly, but keeping the Kremlin on its toes and embarrassed is good. The longer Ukraine holds on, the more equipment they get, like F-16s, Abrams, and ATACMS.