I like slow - Tai Chi slow - at first; progressing to standard speed as practice progresses; when possible.
I was working with a newer student on Sunday; the technique I was trying to teach him was katate-tori ikkyo. Wasn't doing too well; he was trying to muscle through the technique like any other student.
So, I had him do the technique solo in extreme slow-mo, (each repetition taking about 2.5 minutes) to get the 'feel' of the movement. First time I'd really tried it with a genuine student; I was surprised how well it worked. Once he got the dancelike flow down; I added myself in as uke, and over successive repetitions gradually increased the speed until he was doing it at normal practice speed. He did very well; with no further problems - for that technique anyway - with muscling.
My 2 cents - hope it helps.