When objects are built and saved using the part tree, the file is a "recipe" for the part, and TC rebuilds it following the order and parameters of the part tree when it's opened. That's what takes a while here, and by "a while", I mean that my uninspiring six-year 1.4GHz Celeron with the sole benefit of 8Gb RAM takes five minutes to rebuild it, but I'm old and patient, and that leads me to reply that I don't think it's got "troubles". I rebuilt the basis of your part over your file, but saved the new construction. I've just saved it back in 2015 to V21, and it opens with parts sound and part trees intact. I think that the fairly acute relationship of circle to polygon in your original file puts a lot of expectation on the loft tool, even with guide curves. My preference, which has seen TC give me pretty good returns over the years, is to loft through an intermediate profile or profiles, giving a more obvious indication of the way the loft tool should surface the shape. Transitions between polygons and curved profiles are especially subjective, and I've a fair bit of sympathy for the programmers who create a kernel to try to satisfy as many people as possible, with a high possibility leaving many less than satisfied.