But its taken 2.5 years for Russia to gain that ground
Well, strictly speaking, most of that ground was taken in approximately a week.
After the first week or so, Russia controlled a bit less than a third of Ukraine. Following Ukraine's counteroffensives in Kiev, Kherson, and Kharkov the percentage dropped to the current 18% (roughly).
All the changes to the frontline since then have been calculated in dozens and hundreds of sq km only.
Once the front lines were settled, and there became two walls of men facing off against each other - movement largely stopped. This is what's happening in Kursk now... a front line is slowly shaping up. Ukraine took as much as it could in the initial days of confusion, but everything further will have to come at a much higher cost.
This doesn't really put Russia's efforts into any kind of better light - I'm simply nitpicking a little here.
It hurts my brain to remember how fast the gains were in the first week. For a moment, I was sure Nikolayev and then Odessa would fall. There were Russian tanks standing at cross streets in downtown Kharkov. All of Kiev's suburbs were battlegrounds.
I barely slept during that first week.