Demo RuntimeError: function signature mismatch

**Describe the bug**
Throws error when trying to add `"-preset", "veryfast"`
`worker.js onmessage() captured an uncaught exception: RuntimeError: function signature mismatch`

**To Reproduce**
Using your demo section at https://ffmpegwasm.github.io/#demo
use the following arguments, and the error should be reproducible.
```js
{
args: ['-i', 'video.avi', "-preset", "veryfast", '-c:v', 'libx264', 'video.mp4'],
inFilename: 'video.avi',
outFilename: 'video.mp4',
mediaType: 'video/mp4',
}
```

**Expected behavior**
Should just encode the file. Tested on ffmpeg locally via CLI and arguments are valid.

**Screenshots**
https://i.imgur.com/UVFTbLU.png

**Desktop (please complete the following information):**
- OS: [Windows 10 2004]
- Browser [Chrome]
- Version [87.0.4280.66]

**Additional context**
Just as a note, encoding is very very slow. Is this normal?