> they aren't substantially more difficult than other Jazz changes
IMO it's not only the tempo. It's also the fact those changes kind of subvert any feeling of tonality, so you can't really rely on tonal formulas (i.e. cadences) when improvising, so you need to come up with other ways to structure your improv, while constantly shifting between modes/scales/patterns. Becoming fluent in this kind of context is a huge challenge, even in slow tempos.
IMO it's not only the tempo. It's also the fact those changes kind of subvert any feeling of tonality, so you can't really rely on tonal formulas (i.e. cadences) when improvising, so you need to come up with other ways to structure your improv, while constantly shifting between modes/scales/patterns. Becoming fluent in this kind of context is a huge challenge, even in slow tempos.