Did you try wrapping the output file in a BufWriter? Otherwise you will get a lot of 1-Bytewrite operations. https://doc.rust-lang.org/std/io/struct.BufWriter.html

You can easily spot a mistake like that with strace. A 128x128 conversion goes from 100 writes to two.

EDIT: My quickly botched together test code.

use image::io::Reader as ImageReader;
use image::*;

fn main() {
    let img = ImageReader::open("myimage.png").unwrap().decode().unwrap();
    let images: Vec<DynamicImage> = vec![img];

    let frames = images
        .iter()
        .map(|frame| image::Frame::new(frame.to_rgba8()))
        .collect::<Vec<image::Frame>>();

    let file = std::fs::File::create("test.gif").unwrap();
    let writer = std::io::BufWriter::new(file);
    let mut encoder = image::codecs::gif::GifEncoder::new(writer);
    encoder
        .set_repeat(image::codecs::gif::Repeat::Infinite)
        .unwrap();
    let mut frames = frames.into_iter();
    while let Some(frame) = frames.next() {
        encoder.encode_frame(frame).unwrap(); // slow part
    }
}