The original Java code for bucket fill is very fast, sub-second fills in most cases however after converting the code to JavaScript the performance gets worse in some specific cases (Bezier offsetting/flattening combined with Boolean operations for non-closed or transparent shapes). There are some advanced math and algorithmic tricks used in the Lasso tool to speed up exactly the same class of problematic cases in JavaScript. Bucket fill is however more complex which doesn't allow straightforward reuse of these algorithms; additional algorithms are currently being developed to deal with it.

Good luck with your nice paint app! ;-)