Yes it will! Every setup, regardless of power, has a performance limit. Utilising Boxy in your workflow on lower grade hardware allows you to be smarter with your scenes, and get the best renders you can with what you have!

Of course you can, on a per mesh basis. However, there are no solutions out there for batch conversion! Boxy provides a solution to toggle entire collections (potentially thousands of individual meshes) between bounding boxes and meshes. It also offers a solution to toggle visibility in renders and the viewport, in a significantly more optimised UI than Blender's standard outliner.

Simple answer: Each polygon on screen takes up valuable resources, and increases load on the CPU. Using bounding boxes means that, although the mesh is still in the scene, it isn't required on screen therefore the CPU load is significantly reduced. The only time all polygons are required is during the rendering process!