Great question!
Both plugins aim to solve the same problem, but there are some subtle differences.
The other plugin started with a completely different approach but in version 2.0 copied what this plugin does with using PHP files for storing translations. However, Performant Translations goes a step further and uses an entire new translation library to make things even faster and use much less memory. A variation of this library is now being prepared fir direct inclusion into WordPress core itself.
