What kind of hang? The first time you render an 8k texture in a Poser session, the conversion to a mip map will take a while and the progress bar doesn't move during that time. But that happens only the first time, in all following renders this doesn't happen any more.

Also, do you remember if texture filtering was on or off? Large textures without filtering will be much slower than with filtering (I'm talking about orders of magnitude here), because unfiltered lookups don't work very well with caching.