ffmpeg image slideshow timing not consistent

What is the right set of parameters to make all images show for 10 seconds in a slideshow made with ffmpeg?

Using this command on Windows 10 with:

ffmpeg version 4.2.3 Copyright (c) 2000-2020 the FFmpeg developers
ffmpeg -f concat -r 1/10 -i listimages.txt -crf 10 -vf fps=8,format=yuv420p -c:v libx264 imageslides.mp4 -y

Some of the images are showing for 10 seconds as needed, and others are staying visible for over a minute. There are 150 jpg files all the same resolution 1600×1200.

Thanks.