I also implemented a batched client-side web tool to resize photos to a certain file size (in a few hundred lines of JS). It only supports JPEG, but it resizes photos almost instantly https://randomforest.net/resizePhoto.html

Here are a few ideas to make yours faster:

1. Use the browser's canvas element to compress JPEG or PNG images instead of WASM. The libraries which come with browsers can use processor-specific instructions, while WASM only targets the smallest common denominator of the most common architectures, so it will be slower.

2. Which resizing method are you using to find a fitting image size? For me, it worked well enough to downscale by some factor a few times until it fits, but if you want to get super close, you could use binary search.