encode_jpeg benchmarks from the image crate show a bit of slowdown:
name base ns/iter saturate ns/iter diff ns/iter diff % speedup
bench_gray 11,739,248 12,590,876 851,628 7.25% x 0.93
bench_rgb 34,507,766 42,325,700 7,817,934 22.66% x 0.82
rustc 1.23.0-nightly (fa26421f5 2017-11-15)
image 796b65b84
x86_64-unknown-linux-gnu