The rule of thumb is that «и» adds one thought to another, because these thoughts are just related and need to be thought together. In the basic case, the earlier thought is expressed as a clause, and the new thought is expressed as a clause, too, so we have something in the form «Ваня сделал то, и Таня сделала это»; that is the case that corresponds to the English “and”. But actually the earlier thought can be anything, and that thought is often just implied, not expressed.
In that role, the word can be used in a lot of very many places (before verbs, nouns, adjectives, adverbs…), that case you show is nothing special in this regard. In the case we have here, the additional thought is expressed just by a verb, not by a clause: there was slowing down, too. An example for a noun: «Это и Таня сделала». Here, the additional thought is that some statement refers to Tanya, too. The earlier thought was that someone else did that.
My immediate interpretation was: “I even slowed it down to 85% like you told me – that didn't help”. Here, the “earlier thought” is “you told me to slow it down to 85%”. This interpretation doesn't have to be true, because the context, well, is lacking… But I think I explained the mechanics anyway. Look for related thoughts in the situation.