The lion's share of time in RNCryptor are in the calls to PBKDF2. It iterates 20,000 times (10,000 for each key). While it may get a little faster in upcoming versions of RNCryptor, password-based encryption will always be slow. This is a security feature; it's slow on purpose. It's designed to be slow in a way that frustrates attackers while having acceptable impact on the most common use cases.
You can dramatically improve performance by using the key-based, rather than password-based, methods. The key-based methods have no injected slowdowns. This would be my recommended approach if possible. Generate two random 256-bits keys rather than using a password.
You can potentially set the number of PBKDF2 iterations to a smaller number (and that's necessary when dealing with JavaScript for instance), but the faster you make key generation, the worse your security is going to be.
There are a number of ways to modify the format to improve performance for your use case, but it's very easy to mess it up and significantly hurt security. As @Zaph notes, I would either do a lot of study or engage an expert before modifying any security framework.