Computing 2d slice interpolations in the segmentation plugin can take a long time on large images. Computation (according to htop) is done single-threaded only. I don't see why this shouldn't be multithreaded since the only information that is needed to compute a slice is the 'support set' (the manually drawn slices). So it should be possible to compute each slice independently.
If you need example data please let me know (or use the one from T28491 once that bug is solved :-) )

Ubuntu 18.04