This seems like a specific issue you are experiencing. Of course there are ways to optimize import process when using large textures i.e. compression, but it should not be taking an abnormal length of time to import your textures. As suggested earlier, would you mind providing your system specifications?
I have tested importing large texture images (over 4k resolution) and notice it takes about 15 to 20 seconds for it to import. This is expected since it is such a large image file and the Engine has calculate things like the default Texture Group to place it.
If you would like, you can create a new AnswerHub post so we can troubleshoot your issues at a more individual level.