"Better a small number of prayers with concentration, than many without concentration." (Shulchan Aruch OH 1:4) "If... one estimates that he will not be able to concentrate for a large number of prayers, and [therefore] says only a few but with concentration, Hashem considers it as if he... said many prayers with concentration" (Mishna Brurah there)
In practice, I think this means to say vidui (and all of selichot) at the pace which allows you to concentrate best. And don't worry if you end up saying only part of the words. (But make sure what you do say is the important parts, for example I would think that the paragraph ending "aval anachnu veavoteinu hatanu" and a single "ashamnu" paragraph are key things to say)