The plugin default algorithm (using a single packing phase) will provide a decent packing for your UV map, but in most cases it won’t be the most optimal one. If you want to obtain a very tight packing you should enable the Enable Heuristic option in the Heuristic panel. In this mode the packer will be continuously performing many packing iterations (slightly randomizing the process every time), saving the best result it could find. Using this technique you can significantly improve UV map packing in a very short time.
The Heuristic Search Time parameter specifies a time in seconds the plugin will spend on a heuristic search. After timeout is reached the packer will stop and the best result will be applied to the UV map. If the time is set to 0 the packer will perform a search continuously, until you apply the result by pressing ESC.
The plugin can significantly speed up a heuristic search by simultaneously iterating on all packing devices available in your system (CPU and all Cuda-enabled GPUs). In order to use such boost make sure the Use All Devices option in the Packing Devices panel is checked (it should be enabled by default).
If you enable the Advanced Heuristic checkbox, the plugin will use an advanced method during searching. It will examine a broader set of solutions when searching for the optimal one. This method is most useful when packing a limited number of islands, preferably with a greater number of orientations considered - in such case it allows to find a better solution than if using the simple method. Enabling this option is not recommended when packing a UV map containing a greater number of islands.