Why Do Some Mobile Apps Feel So Difficult to Use?

I’ve tried many business apps lately and noticed that some feel smooth and simple, while others become frustrating after just a few minutes. Sometimes the app looks modern, but navigation, loading speed, or usability completely ruin the experience.

I was reading discussions about how businesses improve app performance and user experience while working with a Mobile App Development Agency, and it made me curious about real-world experiences.

In your opinion, what are the biggest mistakes companies make when building mobile apps today?

Would love to hear thoughts from developers, designers, and app users.