gistfile1.txt

(vips) john@banana ~/pics/jxl $ vips --version
vips-8.16.0
(vips) john@banana ~/pics/jxl $ vipsheader -a jxl-layers-e7.jxl 
jxl-layers-e7.jxl: 490x490 uchar, 4 bands, srgb, jxlload
width: 490
height: 490
bands: 4
format: uchar
coding: none
interpretation: srgb
xoffset: 0
yoffset: 0
xres: 1
yres: 1
filename: jxl-layers-e7.jxl
vips-loader: jxlload
n-pages: 18
delay: 67 67 67 67 67 67 67 67 67 67 67 67 67 67 67 67 67 67 
loop: 0
icc-profile-data: 732 bytes of binary data
orientation: 1
bits-per-sample: 8
(vips) john@banana ~/pics/jxl $ time vips copy jxl-layers-e7.jxl[n=-1] x.gif

real	0m1.650s
user	0m4.781s
sys	0m1.014s
(vips) john@banana ~/pics/jxl $ ls -l jxl-layers-e7.jxl x.gif
-rw-rw-r-- 1 john john 462378 Oct 28 10:23 jxl-layers-e7.jxl
-rw-r--r-- 1 john john 261934 Oct 28 10:28 x.gif

Here's the GIF it made: