The Reset function cleans the map, but all operations have a 60 second smooth cleanup duration. This is configurable as the 5th parameter of the destruction functions.
To answer your question about performance, it’s a little complicated. When using encapsulation checking (fast destructions), I only have to worry about divisions along the boundary of the volume, instead of every single division contained within the volume. Consider the difference between the volume of a sphere and its surface area, that is the difference between MeshPart destructions and Part destructions.
TL:DR
Fast Destruction (only with Parts): Scales amazing with part size, you might see your performance increase as it gets bigger. See the first gif in examples where I delete a huge map using a sphere at max size
All other destruction: Scales horribly with part size, because each part must be subdivided all the way down into its constituent voxels