OK, now we're getting somewhere. Could you try a similar thing with something like Stylus or Omnisphere, to see if those players can cope with tempo ramps? If they can. then it is definitely a Kontakt problem, LASS problem, or both. If they can't then the VEP programmers need to have a look at it.
The other thing to look at is whether or not things are better or worse if you change the multi processor settings in Kontakt (I can't remember whether or not you have done this).
DG