If someone bought shares in the SPDR S&P 500 ETF (NYSEMKT:SPY) 15 years ago, they would have more than tripled their money since then. However, the investor would have had to hold through significant drops of nearly 60% in 2009 and 30% in 2020.

To earn great returns, investors have to be willing to stomach and hold through, the volatility of the stock market. Upstart (NASDAQ:UPST) and The Trade Desk (NASDAQ:TTD) both have the potential to create immense wealth, but they will undoubtedly be volatile on the way. If you can buy these two stocks now and hold them forever, you could be much richer a decade or later from now.

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1. Upstart: Redetermining credit

To determine if a consumer is able to get a loan at a good interest rate, a bank traditionally uses just a handful of variables to make a determination. Although not used alone, the primary driver of loan approval interest rates given is the Fair Isaac (NYSE:FICO) FICO score. However, using such a small number of variables can lead to creditworthy consumers getting suboptimal loans. This is where Upstart comes in. It uses over 1,000 variables backtested with over 10 million repayments events to determine creditworthiness. 

Smaller banks often don't have the capital to build the artificial intelligence (AI)-based systems that larger banks have to determine creditworthiness, so they go to Upstart. But, these big banks might want to reconsider their options: In an internal study, Upstart's AI had 75% fewer defaults on its approved loans -- assuming the same approval rate -- than large banks. This has resulted in the number of customers that Upstart has tripling over one year, to 31 banks. 

The main risk for Upstart is its customer concentration. Over 58% of the Q3 loan volume it made decisions on came from one customer. This is extremely high, but it has actually been falling for a while. In the year-ago quarter, the same customer made up 72% of the loan volume, and in 2020 it made up 67% of loan volume. While this major concentration is something that needs to be closely monitored, the rapid adoption Upstart is seeing from other banks is effectively minimizing this threat. 

Despite sales and marketing costs more than tripling year over year, the company pulled in $30 million in net income. Largely, the company's nearly 250% revenue growth is the reason that it is able to hold onto its profits. Upstart has the best of both worlds: It is investing in marketing for the future, growing rapidly today, and it is still bringing 13% of its revenue to the bottom line. What the company has to offer will likely continue to yield success in the long run.

The stock has grown over 300% year to date and nearly 500% over the past year. With Upstart being down 56% off its highs, I think now is a good time to get into this stock if you haven't already. However, this volatility could be telling of its future performance. While I think this stock could perform very well over the long term, it will not come without volatility. Investors should be ready to face severe downturns again, but if you can handle that, today might be your opportunity to invest.

2. The Trade Desk: A market leader that has barely scratched the surface

The Trade Desk has quickly become the leading buy-side adtech platform in the industry, being the main place that advertisers go when they want to place ads online. With The Trade Desk, advertisers can place their ads anywhere from a streaming channel to a podcast. 

The Trade Desk has a huge market ahead of it. The company sees $725 billion being spent on advertising each year, yet only $4 billion in spending went through its platform in 2020. That is room for huge success, but where The Trade Desk really sees its opportunity is in connected TV (CTV). Streaming services offer free or discounted services in exchange for ads, and many consumers prefer to sit through the ads instead of paying a premium. Unless the cost of ad-free streaming declines, the future of streaming will likely be funded in part by advertising, and The Trade Desk plans to capitalize on that. 

CTV advertising is much more effective than any other form of digital advertising. This can be seen in the cost of advertising on CTV devices, where it is twice as expensive to advertise than through other channels. The Trade Desk gets a slice of each transaction, so it is more than willing to encourage CTV advertising. The Trade Desk also has one of the largest CTV reaches, making it the go-to spot for advertisers. It has over 120 million devices and 87 million households under its reach.

The company's dominant role is one that has paid it well. In Q3, the company made $59 million in net income, which grew 43% year over year. This leadership is one that will only get stronger as well. With its reach, advertisers will only want to go to The Trade Desk instead of competitors, which in turn allows the company to invest in bringing more publishers into its reach. This advantage will likely compound, which is why I think every investor should hold onto this company forever. 

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.