We answered this...
In some version of the firmware, Roku added "loadWidth" and "loadHeight" to the poster node . We don't know which version, but using WayBackMachine on this page indicates it was added sometime in the past year. Our images were coming through larger than the bounding boxes and were slowing the UI. For some reason the fonts made the UI completely unusable. Once we scaled the images down using the load* values, then our UI sped up a lot. hopefully this helps the next group of developers who are fighting their way through... [/font][/color][/font][/color]