Why was the end-Permian Extinction So Bad? 

THIS ARTICLE discusses why the recovery from the end-Permian mass extinction was very slow. For almost 5 million years the earth was almost sterile. Recovery from other extinctions was much more rapid. It is thought that the end-Permian, like other extinctions, was caused by lots of carbon dioxide in the atmosphere.

The absence of vegetation removes one source of carbon capture, but that is not the only means of carbon capture. Silicate weathering can also lead to capture of carbon. (As an aside, THIS PODCAST gives an easy introduction to carbon capture.)

But at the start of the Triassic, this did not happen. And this was because the diatoms and radiolaria necessary for this to happen had, along with all life forms, been badly affected by the global warming. It took the Earth a long time to get out of this disastrous cycle.