Does that mean it slowed by double or sped up by double?
Here's a helpful anology: You're waiting in line at a Wal Mart checkout. There are two old ladies with change purses out, and 3 fat ladies with food stamps and an assortment of old gift cards in their hands. The prognosis is that it will serve to double the time it takes you to progress in the line.
I take it to mean that it doubles the time it takes to progress from one stage of the disease to the next. Or from stage one to end stage. Or something similar.
Which is a good thing, IF the effect is real and not imagined. Often it’s the latter, imagined by an overly enthusiastic principle investigator who has stars in his eyes and has been meddling with the data and abusing his poor statistician since the day he hired him, if he ever even bothered to hire one.