Doubling your money over 18 years is a only 4% rate of return. Not terrible, but also not impressive.
$1.1 billion to $4.08 billion [1].
His primary business holding, the Dallas Mavericks, has increased in value from $285 million to $1.9 billion [2] (per the latest Forbes estimate). For that investment, he has seen a rate of value increase of about 11% per year. That investment stomps the S&P 500 massively (which was at ~1400 when the acquisition closed).
[1] https://www.bloomberg.com/billionaires/profiles/mark-cuban/
[2] https://www.forbes.com/sites/kurtbadenhausen/2018/02/07/nba-...
Very few money managers that are accessible to people with less than 10^8 USD can beat the S&P after fees.
Bridgewater, (Ray Dalio's fund mentioned above) regularly gets 12.5% annual returns and they are most notable for being consistent, not high. Pre-2008 some investment banks were getting up to 20%.
A billionaire making 7.5% annually is fairly unimpressive if their goal was to make money (which it might not have been for Cuban).