In general, executing any script adds some overhead. Of course, power of modern computers almost eliminates this issue. Nevertheless, TuneText's creator has made all the best for optimizing the application in terms of speed and size.

The Application is actually a big script, which implements TuneText functions. As the functions are the same for each content page, they are located in separate JavaScript files (about 50K). Because of that, the files have to be downloaded only once, and since then they stay in browser's cache.

In other words, there may be some delay during downloading and opening the first page of TuneText pack. But the rest of the pages are processed fast!

Generally, the TuneText performance depends on number of words in textual content, rather than the file size!
Behind the scene, customization of each word is controlled individually. In turn, these processes heavily depend on performance of the underlying DOM (Document Object Model), the famous standard for dynamic HTML.
For example, each word has unique id. And the DOM's method getElementById scans all available elements on the page every time we need to find a specific word.
The more words, the more time overhead (in milliseconds, of course).

More important, the underlying JavaScript code is highly optimized and tuned as well!