Stocks are off to a hot start in 2019, but a handful of shares are outright smoking. The S&P 500 has managed to climb 11% higher in just the first two months of the year, but there are dozens of stocks that have more than doubled.

Shares of Roku (NASDAQ:ROKU), Cronos Group (NASDAQ:CRON), and Qutoutiao (NASDAQ:QTT) have all more than doubled in 2019. Let's take a closer look at these three highfliers to see why they are blazing with just a sixth of the year in the books.

2019 on a floor mat with the 1 replaced with an arrow pointing up.

Image source: Getty Images.

Roku: Up 116%

Everybody's streaming video these days, and Roku is there as the platform of choice for folks either buying one of its namesake devices or snapping up one of the growing number of smart televisions powered by Roku's operating system. Revenue rose 45% to $742.5 million in 2018, and Roku is expected to top $1 billion in revenue in 2019.

There are now 27.1 million active users streaming through Roku, 40% more than a year earlier. This would be a hollow metric if you didn't know that these users streamed 7.3 billion hours, 69% more than a year earlier. In other words, folks are getting more and more hooked on Roku as their gateway to a plethora of third-party streaming services. Average revenue per user at Roku is naturally also on the rise. 

Qutoutiao: Up 114%

Qutoutiao may not be a household name for stateside investors, but its mobile content platform is gaining traction in China. Qutoutiao lets users aggregate articles and short videos in customized feeds, aided by artificial-intelligence-fueled algorithms. Launched less than three years ago, Qutoutiao has catapulted to become the country's second-most-popular mobile content aggregator in terms of monthly and daily active users. 

Qutoutiao went public at $7 less than six months ago, and it showed the kind of growth it's capable of in its first -- and so far only -- quarterly report as a public company. Revenue surged 520% -- yes, 520% -- to $142.3 million in the third quarter of 2018. Average daily active users have surged 229% to 21.3 million, and the typical user has gone from spending an average of 34 minutes a day on the platform to 56 minutes over the past year. 

We won't have to wait long before Qutoutiao gets to flex its growth muscles. It reports its fourth-quarter results next week. Analysts at UBS and Deutsche Bank downgraded the stock in February on valuation concerns after the heady year-to-date run, giving Qutoutiao a lot to prove come Tuesday's financial update.   

Cronos Group: Up 108%

Marijuana stocks were hot last year, and Canada's Cronos Group capped off the year in style by selling a 45% stake in the company to tobacco giant Altria (NYSE:MO) in a $1.8 billion deal in December. The legalization of recreational cannabis in its home country and the passage of the industrial hemp-friendly U.S. Farm Bill have only helped push the handful of marijuana stocks higher.  

Business is booming. Revenue soared 186% in the third quarter, and that's actually a sharp deceleration from the pace at which it was running earlier in the year. Don't snooze on the Altria purchase. Altria has expertise in tobacco distribution and navigating choppy regulatory waters. It even has a minority stake in a leading vape company. The sluggish Altria may seem to be riding Cronos Group's coattails here, but there will come a time for the tobacco behemoth to return the favor.