We as ATCOs are well aware that going down and slowing down isn't that great, if we give you something impossible just tell us, in this instance "descending FL130, unable to be level by TELTU (at this speed)". Did the controller reissue the "be level by TELTU" when you were recleared to FL130? If not, you do not have to be level and they should have coordinated the descent with the next sector. More importantly, please do not amend your speed issued at all without asking first, even if it is just for a few thousand feet to make a restriction, the trail can be quite tight in that area and separation can be very quickly eroded, sometimes close enough for vortex to be a big issue.