I've seen it done in some maps (Like Firefighters) but I don't know how to do it myself.

How do you make a unit move slower than it does normally?

Use hyper triggers, center a location on that unit, and constantly move another unit above/underneath it, like a burrowed zergling or an observer.

Trigger(Current Player){
Conditions:
"Slow" is set
Actions:
Move 1 zerg zergling for Neutral at anywhere to "Unit Slow Location"
Preserve trigger

If you have hypers it does this about 12x per second, which slows the unit.

In Fire Fighters, Tuxlar could change the amount that the units were slowed by having a certain amount of deaths that needed to be reached before the unit could be moved, of course, for the closer precision he had, he probably used a more complex method, same basic idea though, just reduce the number of times the unit is moved.

Would the number of units being moved affect the units speed?
Is it just how often a unit moves that affects it?

The number of units doesn't matter, it's just the frequency of the movements, which is 8 times per in-game second at the most.

Just 8? Couldn't it be 12 times?
If the trigger cycle runs 12 times a second, then couldn't it move units each trigger cycle?

The trigger cycle runs 11.9 times per real-time second, but 8 times per in-game second.