Question: Are there any aspects to the C++ programming language where the code is known to be slower than the equivalent C language? Obviously this would be excluding the OO features like virtual functions and vtable features etc.

I am wondering whether, when you are programming in a latency-critical area (and you aren't worried about OO features) whether you could stick with basic C++ or would C be better?

Answer: 
Nothing in the C or C++ language standards specifies the speed of any construct (C++ does specify the time complexity of some operations applied to containers, but that's outside the scope of your question). The speed of the code generated for a given construct depends on the compiler used to compile it, and on the system it runs on.

For a given code construct that's valid C and valid C++ with the same semantics, there's no *fundamental* reason why either should be faster than the other. But it's likely that one will be faster than the other if the developers of the compiler were a little more clever.
That depends on what you mean by "equivalent". If you compare C's stdio.h with C++'s iostream, stdio.h operations are generally faster (quite a bit faster in some situations). But if you are talking about code written in the subset of C++ that also compiles as valid C, the generated machine code will likely be identical.