Web performance
This is an underappreciated field that lends to plenty of consulting. As the web becomes more complex, with heavy frontend code, third party services, and demanding amounts of images and videos, it's requiring a deeper understanding about networking and browsers to keep the web performant. Compounding the problem is the growing number of users who use mobile devices to browse the web, which have larger constraints on network and hardware than desktop users.
Additionally, web performance speaks to businesses because a "slow" website can impact your bottom line. There's lots of case studies about this: https://wpostats.com. For large companies with heavily-trafficked websites, this is critical as you're losing potential users/customers as a %.
Google has slowly been making it a bigger objective for websites to focus on SEO-wise, especially with their latest push around "web vitals." Therefore it's becoming a higher priority for marketing departments as well.
Web performance is not sexy and requires a lot of patience around analyzing websites to figure out why they're slow and coming up with specific optimizations for them. This is not like building websites. It's more like debugging. And it requires a pretty deep understanding of how websites load and run in browsers. Provided these things, I think that's partially why it's given so little attention and why so many websites perform terribly. If you're not deterred by these factors, there's a lot of businesses that could use this specialized expertise.