Streaming-TV platform Roku (NASDAQ:ROKU) will have to prove to investors the stock is worth its premium price tag when the company reports second-quarter results early next month. Following a more-than-250% gain in its share price so far this year, Roku now has a pricey valuation, evidenced by its price-to-sales ratio of 14.5 (this compares to Netflix's P/S ratio of 9.3).

Of course, in order to earn a frothy price tag for its stock, Roku has delivered some impressive growth. Revenue has soared and the company's operating margin has expanded. But investors will be looking to see how sustainable its high growth rates are.

To that end, here are three metrics investors will want to check on when Roku reports its second-quarter update.

The Roku Channel displayed on a TV

Image source: Roku.

1. Revenue growth

In the first quarter of 2019, Roku's revenue growth impressively accelerated. The company's top line increased 51% year over year -- up from 46% in the fourth quarter of 2018 and 45% for the full year. This growth was driven by a 79% increase in platform revenue. Roku's platform revenue, or revenue from ads, subscriptions, and transactions on its streaming platform, accounted for 65% of the quarter's revenue. The remaining revenue came from sales of the company's player hardware. This revenue stream rose 18% year over year in Q1.

For its second quarter, Roku forecast revenue to jump 42% year over year to $223 million. Analysts, however, expect revenue to rise 43% and reach $224 million. 

2. Active account growth

Another metric worth keeping an eye on is Roku's active account growth. This metric has seen notable momentum recently, with Roku adding 2 million active accounts in its first quarter of 2019 -- well above the 1.5 million the company added in the first quarter of 2018.

Active account growth in Roku's second quarter of 2018 was weaker than in any other period last year, with net growth in the key metric coming in at 1.2 million accounts. Given Roku's premium valuation and strong active account growth recently, investors should look for this figure to be higher in the second quarter of 2019 than it was in the year-ago period.

3. Ad impression growth

Finally, investors will want to check on Roku's growth in video ad impressions on its platform. Video ad revenue has been a key driver for Roku's platform business, with monetized video ad impressions more than doubling in Q1 and for the full year of 2018 compared to their respective year-ago periods.

In order for Roku to keep up its torrid growth, this key catalyst will need to keep growing rapidly. Investors should look for a 100% or greater increase in monetized video ad impressions once again in Q2.

Roku will report its second-quarter results after market close on Wednesday, Aug. 7.

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.