[Jweb_Guru]

I am afraid your premise is incorrect. Foreign keys can be enforced (particularly on insert-only or insert-mostly workloads) with very little contention, even in a distributed system: http://www.bailis.org/papers/ramp-sigmod2014.pdf

And lest you think there is some hidden constant factor here that destroys real performance at the cost of "scalability," the techniques from that paper were used to completely destroy the TPC-C benchmark: http://arxiv.org/pdf/1402.2237.pdf

Admittedly this stuff is not yet implemented in traditional RDBMSes, but there is absolutely no reason why it could not be and I fully expect it to in the future. Certainly, if you are having performance problems now, this knowledge doesn't help. But I'd venture to guess (from personal experience) that most people removing foreign keys are doing it preemptively, rather than evaluating whether it is actually causing performance problems.