Shares of edge computing company Fastly (FSLY -0.54%) are back at $40 as of this writing on Thursday. The threshold is a somber level for Fastly bulls as it represents more than a 50% decline from where the stock traded at the beginning of the year. It's been a painful 2021 for Fastly investors, to say the least.

But is the stock now trading low enough to be an attractive buy? Based on the company's business momentum in the first half of 2020, yes. But in light of the company's performance since then, the stock may not be as attractive as it might seem.

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The 2020 Fastly vs. the 2021 Fastly

During the first half of 2020, Fastly's business was firing on all cylinders. Revenue for the six months ended June 30 was $137.6 million, up an impressive 50% year over year. The tech company had also just wrapped up a quarter in which revenue growth accelerated to 62%. Average enterprise customer spend on its platform was $716,000, up from $642,000 just three months earlier.

"The structural and societal changes resulting from the global pandemic continues to significantly accelerate the need for organizations to prioritize their digital transformation," said Fastly CEO Joshua Bixby in the company's second-quarter shareholder letter.

On the heels of so much momentum, management lifted its guidance for full-year revenue. The company said at the time that it expected revenue between $290 million and $300 million, up from a previous forecast for revenue of $280 million to $290 million.

Things didn't pan out quite as well as expected. Fastly's 2020 revenue did end up coming in at its guidance range. But it wouldn't have if it wasn't for a small acquisition later during the year. Total 2020 revenue was $291 million -- and revenue growth decelerated meaningfully from Q2 levels. It turned out that Fastly lost its biggest customer: ByteDance, the owner of TikTok. This meant Fastly would roll into 2021 going up against some tough year-ago comparisons that included both elevated internet usage as consumers sheltered at home and revenue generated from the fast-growing TikTok social media platform.

With such a tough comparison (plus a short platform outage during Q2 that negatively impacted revenue), revenue during the first half of 2021 only grew 23% year over year during the first six months of 2021.

Don't count Fastly out

While it's been a tough 2021 so far for Fastly, investors shouldn't count the company out. After all, it is still a growth stock. Management guided for revenue to be between $340 million and $350 million this year, well above the $291 million in revenue the company reported last year. Moreover, Fastly is still seeing spend from existing customers grow meaningfully year over year.

But investors should realize that with worse-than-expected top-line momentum and regular profitability still seeming a ways out, Fastly shares aren't necessarily a bargain just because they are trading substantially lower. Investors should look for more proof from management that this growth stock is worth a high premium before they turn bullish on this stock.

So don't count Fastly stock out. But don't get too excited yet, either. Investors should watch the company closely and be patient about building a position.