Good question and this is directly linked to what I said here:
In LBT, we do the ray tracing for a full sphere around each point so that we can account for the radiation that’s “reflected” off the ground (technically, we’re doing this by assuming an emissive ground hemisphere that has 0.2 the intensity of the sky hemisphere, much in the same way Radiance does). In legacy, we only looked at the radiation coming from the sky dome, which was ok for some purposes but could be 20% less than expected for vertical wall faces. Particularly when we had people trying to use Ladybug Radiation studies to inform HVAC sizing and peak cooling, a margin of error that big just isn’t going to cut it.
Long story short, in LBT, we use twice the number or rays (modeling both ground and sky hemispheres) and so it takes twice as long.