People should say Kaddish together. Halachically, according to Rav Moshe Fienstein, ZT”L, anything said aloud in a Shul should follow the Minhag of the Shul, i.e., so unless it is a Sfardi Shul, their longer Kaddish should not be said, though people do it anyway. The same thing applies to Kedusha.
I find that in most cases, the people saying Kaddish very fast, are either saying it for a non-family member, or being paid to do so and are not saying it for a parent. That, to me, is unfair to those who are saying it with the proper Kavannah to fulfill their own obligation.