Office space firm WeWork, which could be the next big test of the IPO market, is often called a $47 billion company. But investors lately have signaled they think WeWork is worth only about half of that, as questions mount about whether the tech-fueled real estate startup can sustain its breakneck growth.

WeWork’s stock on the secondary market, through which private investors buy stakes from earlier shareholders, has been hovering at levels that would value the firm between $21 billion and $23 billion, according to brokers. That’s a drop of around 30% in six months, and also less than half the level at which it recently raised new funding. Mutual fund backers including T. Rowe Price and Fidelity Investments also have marked down the value of their stakes to valuations between $22 billion and $23 billion.