Developing a cross-platform crypto trading app allows you to reach users on both iOS and Android platforms using one codebase. This reduces time and effort and allows for a unified trading experience across platforms. To achieve this, developers need to choose the right frameworks and follow best practices in terms of performance, security, and user experience.

Choosing the Right Framework
The first thing to do is to select the appropriate cross-platform framework. The most popular ones include:

React Native: It's a JavaScript framework that allows developers to build mobile apps with near-native performance. React Native is extremely popular in the crypto space due to its flexibility, huge community support, and large pool of ready-to-use components.

Flutter: Developed by Google, Flutter uses the Dart programming language and provides highly customizable widgets. It's an excellent option for developing visually appealing crypto trading apps with a smooth user experience.

Xamarin: Founded on the .NET ecosystem, Xamarin uses C# to build iOS and Android applications. It provides a near-native experience and is ideal for developers familiar with Microsoft technologies.

Integrating Real-Time Market Data
For crypto trading apps, real-time data is essential. Developers should integrate WebSocket technology so that users receive live price updates and trading data without latency. REST APIs of exchanges like Binance or Coinbase might provide access to market data, but WebSockets are better for real-time trading applications.

Security Measures
Security is the most important part of crypto trading apps as they deal with sensitive user data and financial transactions. Some of the best security practices include:

Two-Factor Authentication (2FA): Activate 2FA to provide an additional layer of security for user accounts.

Data Encryption: Use end-to-end encryption for user-server communication to protect sensitive data.

Secure API Integration: All APIs, and especially trading APIs, must be secure and provide OAuth or API key authentication support.

User Experience (UX) Design
An intuitive and user-friendly interface is crucial in the competitive crypto market. The app should have intuitive navigation, real-time charts, easy-to-read order books, and smooth execution of buy and sell orders for assets. Adding customizable widgets and real-time notifications can also improve the user experience.

Testing and Optimization
Thorough testing needs to be conducted to ensure that the app performs well on every device. The performance testing needs to include testing the app's ability for high-frequency trading and extremely large numbers of simultaneous users. Load testing and stress testing identify potential bottlenecks in the system. Optimizing the app for quick data access and execution of trades is also vital to a responsive trading experience.

Creating a cross-platform crypto trading app entails choosing the right framework, real-time market data management, high security, and optimizing user experience. By putting these vital considerations first, developers can create an application that not only performs excellently but also captures users' attention and security while trading in cryptocurrencies.