Long-time Roku (ROKU -6.60%) stock bull from KeyBanc Capital Markets, Justin Patterson, isn't so optimistic these days. The analyst lowered his rating on the stock this week, changing it from sector overweight to sector weight -- ratings that are similar to buy and hold, respectively. In addition, the analyst removed its 12-month $72 price target. The move comes after shares have been crushed this year, falling more than 75% as of this writing.

What's behind the analyst's more pessimistic outlook for the company? It boils down to the company simply not performing as well as expected, he explains. In other words, there are indications that the company may be losing market share in the important connected TV market instead of gaining market share.

But is the analyst right? Or is he pulling the plug at the exact time the stock is becoming attractive?

Roku has some serious problems

There's no way to sugarcoat it; Roku's business performance has been poor recently. Its quarterly year-over-year revenue growth rates have slowed throughout the year, with first-quarter growth coming in at 28% and then second- and third-quarter revenue rising 18% and 12%, respectively.

Even more, the results have significantly underperformed management's own expectations. Even as the macroenvironment was rapidly deteriorating earlier this year, Roku management said in an update on April 28 that it still expected full-year revenue to increase 35% despite reporting just 28% growth in Q1 and management guiding for approximately 25% growth in Q2. But management's most recent view for the full year now anticipates total revenue growth of about 11%.

Management is blaming the macroenvironment. But some of Roku's digital advertising peers' strong growth during this same period is raising eyebrows. The Trade Desk (TTD -7.16%), which generates more revenue from connected TV than any other advertising channel on its platform, has seen its trailing-9-month revenue increase 36% year over year. Sports-first streaming TV app fuboTV (FUBO -3.38%) is similarly seeing strong growth, with Q3 ad revenue rising 21% year over year and management saying in its Q3 shareholder letter that it expects

continued strength into the fourth quarter, with increased demand heading into the seasonally strong holiday period augmented by a competitive mid-term election cycle.

This contrasts with Roku's guidance for Q4 revenue to fall about 8% year over year. Even more, management said it is difficult to know when headwinds impacting ad spend on its platform will ease up.

The bull case is becoming too speculative

Roku management, of course, tries to console investors, saying that it believes its current challenges are "temporary." But Roku's underperformance relative to some of its peers begs the question: Is the company's competition tougher than investors and analysts had predicted?

Until we have evidence of Roku delivering exceptional growth again in the connected TV market, it may be wise to avoid buying shares of this former Wall Street darling. A bet that Roku's disappointing business performance is only temporary requires too much speculation at this point. The company will likely need to prove to investors it has a special product capable of grabbing meaningful market share like it used to before Wall Street buys into this growth story again.