Depends on your lag how far you get.
The more lag, the faster the lava expands even if it takes you forever to clear one dirt clog. Basically the lava isn't taking into consideration your lag, so it'll keep expanding at the same rate, even if your digger is slowed to a crawl.
When my browser was very laggy I was getting around in the 50s. Now that it's not lagging so much after a browser update I'm getting around in the 100s. I imagine to get the 150 or 200 medal requires even less lag.
Reminds me of some of the badly-coded games of the past, where the timer was realtime even if it wasn't physically possible to do on slower machines. Any timing function (in this case lava expansion rate) should be tied to the actual rate you can move, not tied to realtime.
It makes it not possible to do if your computer is too fast/slow. There actually exists at least one DOS game which cannot work on a computer slower than a certain amount (because one level becomes impossible in that scenario), nor can it work on a computer faster than a certain amount (because a different level then becomes impossible). That isn't the kind of coding style you want to emulate.
The only thing lag should do is slow everything down annoyingly, not make games literally impossible due to badly-coded timings.