As a matter of interest we didn't get a full list of the key words until the summer holidays at the end of Yr R and I didn't even know their significance so we just ignored them over the summer hols. DS1 only had 23 key words at the end of YR R as a result.
Once I worked out their significance, at the beginning of Yr 1) we had done all 400 done by the Easter of Yr 1 but that was through practicising them daily. However, it worked well for us - not sure all children could cope with learning that way though as it was very dry but the advantage was he could see what he was aiming at and how he was improving. The teacher pushed him up through the levels of the ORT as we went along. He reads very very well now.
I would be worth asking for a list of the first 45 key words they are supposed to know in YR R (or however many your school has for Yr R - our school had 100 but I think 45 is the minimum). You have to do reading practice with the words in context too but learning those keywords makes all the difference.