This mod moved! The speed sliders are now part of Pyanodon's Better Caravans (https://mods.factorio.com/mod/py-better-caravans) together with my hitbox fix and Idle Caravans. Speed is now set in the game: open the Better Caravan settings window from the shortcut bar and type your old value once.

This last version is empty and just pulls in the new mod. Once it's installed you can remove this one.