Not really. In practice, having a multi-core CPU will be faster than individual single-core CPUs. Multi-core CPUs essentially take a single-core CPU, duplicate it several times, and put it all on one piece of silicon. Inter-core communication which is required for multithreaded programs is faster as the communication never leaves the die and is thus significantly lower in latency and generally much higher in bandwidth as well. Figure about a 5-10x reduction in latency with inter-core communication vs. communication between different CPUs in different sockets. Plus with modern CPU turbo modes, you can essentially hit the same single-threaded clock speeds with a bunch-of-cores CPU vs. a theoretical CPU with only one core.
The only disadvantage you'd get with having a single multi-core CPU vs. several single-core CPUs would if and only if you had a modern IMC and point-to-point bus (HT or QPI) setup and needed maximum aggregate memory capacity and bandwidth. Having more memory controllers with multiple CPUs would do that. But that's a real edge case and if you fit in that edge case, both Intel and AMD have multi-socket setups with low-core-count (4 core) CPUs to sell you (e.g. Opteron 6308 and Xeon E5-2637 v3). The fact they sell very few of those compared with the 12/16/18-core units is very telling of just how much of an edge case it is.