If the feature is so easy to implement that Skype can roll it out in a couple of weeks, it's probably not a sustainable model to build a business on. There would be no "economic moat"[1] to protect your business against competitors. Eventually, your competitors would figure out how you implemented your algorithm and implement it themselves.

Also, even if FastChat was the only app with your new feature (maybe because you were able to patent it), that still wouldn't guarantee success. Skype, Zoom, etc. would already have a huge base of customers while FastChat would be starting with none, so they'd have the advantage of network effects (a FastChat user would first have to convince someone they wanted to talk to to install FastChat). They'd also have much more resources to throw at alternate approaches to making their apps faster, and they'd probably be much more reliable than FastChat because they'd have large operations staffs to keep their servers running under high loads. Corporate users would probably want to stick with a service with a proven track record, since this kind of service is now mission-critical infrastructure for them.

[1] https://www.investopedia.com/ask/answers/05/economicmoat.asp