When fly fishing for trout on lakes, it's best to always vary your retrieve to begin with in order to find out what the trout prefer. I tend to start with a slow figure of 8, then move into short slow strips, then longer and faster. Eventually, I will mix all of them into one retrieve if I need to. When retrieving, remember that a slower retrieve keeps your flies deeper while a faster retrieve brings your flies up shallower in the water column.