If a group of monsters were, say, Slowed and then subjected to Sickening radiance would their speed be one-half normal, or one-fourth?
Slow has, in part:
An affected target’s speed is halved...
A monster who fails their Sickening radiance saving throw:
...suffers one level of exhaustion...
...which will halve its speed when the second level of exhaustion is reached.
Effects don't stack with themselves, but I'm unclear if the halving of speed is the effect, or if different sources of speed halving will apply together.