Focusing may be slow or even interrupted because, by default, Biolucida (Research) loads new image data every time you move to a different plane. To minimize this problem, use the Pre-load current view tool to load every image plane in the stack at the highest possible quality prior to focusing.
Biolucida (Research) loads from the top of the stack downwards, and keeps loading planes until the bottom of the stack is reached or until you click Stop.
Biolucida (Research) loads image data at the highest resolution for every focal plane UNLESS Disable higher qualities is selected under movement resolution.