Shares of Roku (NASDAQ:ROKU) have been on an absolute tear. With the help of Roku's better-than-expected first-quarter results earlier this month, the stock is up a wild 169% year to date and 135% over the past 12 months.

The impressive performance may have some investors wondering what has been sparking so much optimism for the stock. To get a better idea of the drivers behind Roku's growth, here's a review of some of the key metrics from the company's first-quarter earnings release that have been making the streaming-TV platform stand out.

A woman on a couch eating popcorn while watching TV

Image source: Getty Images.

1. 51% revenue growth

First, and foremost, there's Roku's fast-growing top line. Revenue in Roku's first quarter soared 51% year over year -- a meaningful acceleration compared to 45% growth in the fourth quarter of 2018. 

2. 79% platform revenue growth

Accounting for 65% of total revenue, Roku's platform revenue continues to be the company's main growth driver. Defined as revenue from the monetization of its platform through advertising and the company's share of third-party subscription and transactional revenue for content on its platform, Roku's business model is built primarily on growing this segment; it is not only larger and faster-growing than Roku's player segment, but it boasts a 70% gross profit margin compared to 10% for its hardware sales.

Highlighting the strength of this segment, a 79% year-over-year increase in first-quarter platform revenue was a slight acceleration compared to 77% growth in Q4.

3. 18% player revenue growth

But investors shouldn't count Roku's player segment out. Revenue from the business, which consists of sales of the company's streaming players, increased 18% year over year in Q1.

Healthy growth in this segment is important, as it means Roku is getting its platform into the hands of more users and ultimately creating more monetization opportunities.

4. 60% gross profit growth

Capturing how lucrative Roku's business model is, the company's gross profit increased to $100.9 million in the first quarter of 2019, up 60% year over year. Notably, this gross profit was on just $206.7 million in revenue, giving Roku a consolidated gross profit margin of 48.8%, up from 46.2% in the year-ago quarter.

5. 74% more streaming hours

Roku's 29.1 million active accounts (up from 20.8 million in the year-ago period) are spending a lot of time on the platform. Total hours streamed during Q1 were 8.9 billion, up 74% from 5.1 billion in the prior-year quarter.

6. $10 million of adjusted EBITDA

Going into Roku's first quarter, management said it expected adjusted earnings before interest, taxes, depreciation, and amortization (EBITDA) to be between negative $12 million and negative $8 million. But the actual result crushed these expectations, coming in at positive $10 million. This was up from negative $0.8 million in the year-ago quarter.

As a result, management lifted its guidance for full-year adjusted EBITDA; Roku now expects the adjusted profit figure in 2019 to be between $10 million and $20 million, up from a previous forecast for about breakeven.