One of the toughest rules of thumb to follow in investing is to let your winners run. With people calling for you to take your profits off the table and worrying about where things are going, staying the course is hard.

Because good companies can continue performing well for extended periods of time, pulling your money out of a stock can seriously undermine your total returns over the long haul.

The two stocks below show how well patient investors were rewarded simply for being smart enough to do nothing and hold on last year.

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Roku: Up 337% in 2019

Streaming video was a growing business even before the pandemic started keeping people at home more. At the end of 2019, Roku (NASDAQ:ROKU) had 37 million accounts through which people accessed their favorite channels and shows. Over 40 billion hours of content were streamed through Roku last year, leading average revenue per user (ARPU) to spike 9% to $23.14.

Roku also set itself up to have its technology in new smart TVs. One out of every three smart TVs sold in the U.S. last year was a Roku smart TV, according to the company.

Having created that ecosystem, advertisers and content producers are clamoring to get on Roku. It's been several years since hardware was Roku's primary source of revenue; advertising dollars are now its main income stream. Last year, platform revenue increased 78% to $741 million, or two-thirds of the $1.1 billion it generated in total.

That kind of outsize performance caused investors to rush into Roku's stock, leading it to more than quadruple in value in 2019, and the bullish sentiment persists. Because the streaming platform controls 30% of the advertising that companies bring to it, there's a good likelihood Roku will continue expanding. Under its advertising model, channels set up their own ad server and send 30% of inventory to Roku. If Roku sells ads in that space, it keeps 100% of the revenue.

The tech stock is up another 40% so far in 2020, a fairly significant achievement considering the recent market turmoil. Although some might think Roku should have done better than that, since streaming video became even more central to our lives during the lockdown, keep in mind that advertising budgets were also impacted.

Even so, Roku now has 43 million active accounts; 14.6 billion hours of content were streamed in its fiscal second quarter, up 2.3 billion hours from the first; and ARPU rose 18% to $24.92 on a trailing 12-month basis. All of which suggests the stock will keep doing well.

ROKU Chart

ROKU data by YCharts

Carvana: Up 181.4% in 2019

Used car dealer Carvana (NYSE:CVNA) had a nearly six-year run of revenue doubling year over year every single quarter, only to see it snapped in last year's fourth quarter with "just" an 89% increase. However, even with the COVID-19 outbreak causing massive levels of unemployment, Carvana was still able to grow, reporting revenue growth of 27% over the first six months of 2020.

It's unique car-selling model that served it so well before the pandemic became vital during the shutdown that saw car dealerships closed. Because Carvana sells its cars online, handling the entire process in literally minutes compared to the hours it can take at a traditional dealership, and then delivers the car to the buyer, it has been able to maintain growth even in this difficult period.

Now that people are heading back to work as businesses reopen, they'll have cash again to buy a car, a purchase they may have put off during the lockdown. Carvana offers consumers a like-new car at a better price than new and this points to further growth potential.

Certainly its stock has maintained its upward trajectory. After nearly tripling in value in 2019, Carvana's stock is up another 125% over the first nine months of this year. That makes its total gain for the last 21 months more than 535%; had an investor sold out early, he or she would have missed tremendous upside.

Carvana is still producing losses, which could make its stock more volatile, but its contactless car-selling business model could be the wave of the future, and that could pay off.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.