First of all I know it is normal to take long to import the textures especially if they are large but I have noticed two things.
The process is single threaded i.e. it uses around 25-30 % of the CPU
Q1: Isn’t there a way to use more of the CPU to do this.
Q2: On second thought isn’t there a way to force the process to done on the GPU instead? It would be much faster
The DDS cube map is exported by SpaceScape for Unreal
Although I did try with the NVidia cube map plugin. With The following configuration. The pictures were 2048 in size and the format was unsigned ARBG 32bpp. No mip map generation