Slow Play Rules is an optional change for FIST to encourage longer missions.
The PDF is two pages, one of which is bare text and the other is the FIST compatability logo. Everything is straightforward and easy to read.
Slow Play's core changes are pretty comprehensive, covering everything from War Dice to item uses and ability uses. They're also written in such a way as to play well with other add-ons, so using Slow Play won't invalidate other expansions.
Overall, if you want FIST missions to feel a little more like a full session of a more conventional ttrpg, I'd give Slow Play a look. It's quick, direct, and comprehensive.