They should be able to be paused, controlled for prev/next movement, last a minimum of 3 seconds (and longer for large pieces of text). If it is too slow the user can click "next" without it being an annoying experience. If it is too fast - even with the ability to go back - it is an annoying experience without the ability to pause it. But it is always better to be too slow than to be too fast.
My honest advice would be [1] and it should be designed in such a way that all reviews are visible, perhaps in 3 columns, instead of a stupid carousel slider. But slowing down the slider would be more trivial - as I expect very few people are actually able to read all of the text of any of the excerpts before it changes on them.
[0] https://www.w3.org/TR/UNDERSTANDING-WCAG20/time-limits-pause...