I have used both rump and neck fillets extensively over the years. Rump cut into suitable bite sized pieces will certainly cook in no time at all and is my preferred choice for kebabs for this reason. Neck fillet can vary, I have had very good quality fillet from butchers that does cook very quickly but from supermarkets, I generally find that it needs a long slow cook.
Mind you, I also wonder at times as to whether what supermarkets say is neck fillet is actually, shoulder fillet. ASDA changed labels almost over night from neck fillet to shoulder fillet, yet the meat within the packet looked very much the same. Buyer beware.