What language should I use for a PDF editor with handwriting?

What part feels slow? Drawing pen strokes, rendering a page, opening the file and saving changes are different problems. Try timing those operations separately, then profile the slow one before committing to a rewrite.

Also test the PDF library on its own. Many Python libraries use native code for expensive work, so the bottleneck might be how often your app calls the library or redraws the page rather than Python itself.

If rendering turns out to be the bottleneck, you don't necessarily have to throw away the Python app. You could keep the interface and move just that part to a lower-level implementation or use a different rendering library. Measure again afterward to see whether it actually helped.

Rust is an option if you want a compiled language and don't mind learning it alongside the PDF work. I'd choose a suitable PDF library first, though. Rewriting an inefficient operation in Rust won't fix the underlying algorithm, and building a PDF editor from scratch is a substantial project.

It may help to narrow down what editing means here. Adding pen marks and text is a different scope from modifying existing text in a PDF. And if you want to convert handwriting into typed text, that's a separate recognition problem. Pick libraries that support the features you actually need before picking the language around them.